The CD2 adhesion molecule is a cell surface protein expressed by T cells and natural killer cells. CD2's extracellular domain contains immunoglobulin-like domains that are glycosylated at two sites and can mediate homodimerisation []. Ligation of CD2 by CD58 in humans or CD48 in mice helps T cells adhere to antigen-presenting cells, and initiates signal transduction pathways that enhance signalling through the T cell receptor for antigen. CD2 knockout mice exhibit essentially normal immune function [], and it is thought that CD2 is somewhat functionally redundant with other T cell co-stimulatory receptors such as CD28 [].
CD2BP2, also known as U5-52K or LIN1, is a component of the U5 snRNP complex and acts as a splicing factor [, ]. It also may play a role in the immune response as CD2 receptor binding protein 2 (CD2BP2) []. Lin1 from Saccharomyces cerevisiae is a non-essential component of U5 snRNP []; it may link together proteins involved in chromosome segregation, mRNA splicing and DNA replication [].
The ferrous iron efflux (FieF) proteins are members of the cation diffusion facilitator (CDF) family of membrane-bound transporters. These are divalent metal cation cation transporters that, in addition to their involvement in iron detoxification in Escherichia coli, FieF proteins are also able to transport Zn2 and Cd2 in a proton-dependent manner [].
CD2AP is a multifunctional adaptor protein involved in membrane trafficking, dynamic actin remodelling and endocytosis [, ]. CD2AP contains three SH3 domains, a proline-rich region, and a C-terminal coiled-coil domain. Different biological processes regulated by CD2AP require specific binding partners [].This entry represents the first SH3 domain (SH3A). It binds to the PXXXPR motif present in c-Cbl and the cytoplasmic domain of cell adhesion protein CD2 [].
The basic structure of immunoglobulin (Ig) molecules is a tetramer of two light chains and two heavy chains linked by disulphide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains: alpha, delta, epsilon, gamma and mu, all consisting of a variable domain (VH) and three (in alpha, delta and gamma) or four (in epsilon and mu) constant domains (CH1 to CH4). Ig molecules are highly modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. The domains in Ig and Ig-like molecules are grouped into four types: V-set (variable; ), C1-set (constant-1; ), C2-set (constant-2; ) and I-set (intermediate; ) []. Structural studies have shown that these domains share a common core Greek-key β-sandwich structure, with the types differing in the number of strands in the β-sheets as well as in their sequence patterns [, ].Immunoglobulin-like domains that are related in both sequence and structure can be found in several diverse protein families. Ig-like domains are involved in a variety of functions, including cell-cell recognition, cell-surface receptors, muscle structure and the immune system []. This entry represents C2-set domains, which are Ig-like domains resembling the antibody constant domain. C2-set domains are found primarily in the mammalian T-cell surface antigens CD2 (Cluster of Differentiation 2), CD4 and CD80, as well as in vascular (VCAM) and intercellular (ICAM) cell adhesion molecules.CD2 mediates T-cell adhesion via its ectodomain, and signal transduction utilising its 117-amino acid cytoplasmic tail []. CD2 displays structural and functional similarities with African swine fever virus (ASFV) LMW8-DR, a protein that is involved in cell-cell adhesion and immune response modulation, suggesting a possible role in the pathogenesis of ASFV infection []. CD4 is the primary receptor for HIV-1. CD4 has four immunoglobulin-like domains in its extracellular region that share the same structure, but can differ in sequence. Certain extracellular domains may be involved in dimerisation [].
CD59 (also called 1F-5Ag, H19, HRF20, MACIF, MIRL, P-18 or protectin) inhibits formation of membrane attack complex (MAC), thus protecting cells from complement mediated lysis. It has a signalling role, as a GPI-anchored molecule, in T cell activation and appears to have some role in cell adhesion through CD2 (controversial). CD59 associates with C9, inhibiting incorporation into C5b-8 preventing terminal steps in polymerisation of the (MAC) in plasma membranes. Genetic defects in GPI-anchor attachment that cause a reduction or loss of both CD59 and CD55 on erythrocytes produce the symptoms of the disease paroxysmal nocturnal haemoglobinuria (PNH). A variety of GPI-linked cell-surface glycoproteins are composed of one or more copies of a conserved domain of about 100 amino-acid residues [], []. Among these proteins, U-PAR contains three tandem copies of the domain, while all the others are made up of a single domain.As shown in the following schematic, this conserved domain contains 10 cysteine residues involved in five disulphide bonds - in U-PAR, the first copy of the domain lacks the fourth disulphide bond.+------+ +------------------------+ +---+| | | | | |xCxxCxxxxxxCxxxxxCxxxxxCxxxxxxxxxxxxxxxxxxCxxxxCxxxxxxxxxxxxxxCCxxxCxxxxxxxx| | | |+---------------------+ +--------------+'C': conserved cysteine involved in a disulphide bond.
The glycine-tyrosine-phenylalanine (GYF) domain is an around 60-amino acid domain which contains a conserved GP[YF]xxxx[MV]xxWxxx[GN]YF motif. It was identified in the human intracellular protein termed CD2 binding protein 2 (CD2BP2), which binds to a site containing two tandem PPPGHR segments within the cytoplasmic region of CD2. Binding experiments and mutational analyses have demonstrated the critical importance of the GYF tripeptide in ligand binding. A GYF domain is also found in several other eukaryotic proteins of unknown function []. It has been proposed that the GYF domain found in these proteins could also be involved in proline-rich sequence recognition [].Resolution of the structure of the CD2BP2 GYF domain by NMR spectroscopy revealed a compact domain with a β-β-α-β-beta topology, where the single α-helix is tilted away from the twisted, anti-parallel β-sheet. The conserved residues of the GYF domain create a contiguous patch of predominantly hydrophobic nature which forms an integral part of the ligand-binding site []. There is limited homology within the C-terminal 20-30 amino acids of various GYF domains, supporting the idea that this part of the domain is structurally but not functionally important [].
ITK (also known as Tsk or Emt) is a member of the Tec family, which is a group of nonreceptor tyrosine kinases containing Src homology protein interaction domains (SH3, SH2) N-terminal to the catalytic tyr kinase domain. It also contains an N-terminal pleckstrin homology (PH) domain, which binds the products of PI3K and allows membrane recruitment and activation [], and the Tec homology (TH) domain, which contains proline-rich and zinc-binding regions. ITK is expressed in T-cells and mast cells, and is important in their development and differentiation [, ]. Of the three Tec kinases expressed in T-cells, ITK plays the predominant role in T-cell receptor (TCR) signaling. It is activated by phosphorylation upon TCR crosslinking and is involved in the pathway resulting in phospholipase C-gamma1 activation and actin polymerization []. It also plays a role in the downstream signaling of the T-cell costimulatory receptor CD28 [], the T-cell surface receptor CD2 [], and the chemokine receptor CXCR4 []. In addition, ITK is crucial for the development of T-helper(Th)2 effector responses []. This entry represents the SH3 domain of ITK.
CIN85, also called SH3 domain-containing kinase-binding protein 1 (SH3KBP1), CD2-binding protein 3 (CD2BP3) or Ruk, is an adaptor protein that is involved in the downregulation of receptor tyrosine kinases by facilitating endocytosis through interaction with endophilin-associated ubiquitin ligase Cbl proteins [, , ]. It is also important in many other cellular processes including vesicle-mediated transport, cytoskeletal remodelling, apoptosis, cell adhesion and migration, and viral infection, among others []. CIN85 exists as multiple variants from alternative splicing; the main variant contains three SH3 domains, a proline-rich region, and a C-terminal coiled-coil domain. All of these domains enable CIN85 to bind various protein partners and assemble complexes that have been implicated in many different functions. This entry represents the first SH3 domain (SH3A) of CIN85; SH3A binds to internal proline-rich motifs within the proline-rich region. This intramolecular interaction serves as a regulatory mechanism to keep CIN85 in a closed conformation, preventing the recruitment of other proteins. SH3A has also been shown to bind ubiquitin and to an atypical PXXXPR motif at the C terminus of Cbl and the cytoplasmic end of the cell adhesion protein CD2 [].
The basic structure of immunoglobulin (Ig) molecules is a tetramer of two light chains and two heavy chains linked by disulphide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains: alpha, delta, epsilon, gamma and mu, all consisting of a variable domain (VH) and three (in alpha, delta and gamma) or four (in epsilon and mu) constant domains (CH1 to CH4). Ig molecules are highly modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. The domains in Ig and Ig-like molecules are grouped into four types: V-set (variable; ), C1-set (constant-1; ), C2-set (constant-2; ) and I-set (intermediate; ) []. Structural studies have shown that these domains share a common core Greek-key β-sandwich structure, with the types differing in the number of strands in the β-sheets as well as in their sequence patterns [, ].Immunoglobulin-like domains that are related in both sequence and structure can be found in several diverse protein families. Ig-like domains are involved in a variety of functions, including cell-cell recognition, cell-surface receptors, muscle structure and the immune system []. This entry represents the V-set domains, which are Ig-like domains resembling the antibody variable domain. V-set domains are found in diverse protein families, including immunoglobulin light andheavy chains; in several T-cell receptors such as CD2 (Cluster of Differentiation 2), CD4, CD80, and CD86; in myelin membrane adhesion molecules; in junction adhesion molecules (JAM); in tyrosine-protein kinase receptors; and in the programmed cell death protein 1 (PD1).
The glycine-tyrosine-phenylalanine (GYF) domain is an around 60-amino acid domain which contains a conserved GP[YF]xxxx[MV]xxWxxx[GN]YF motif. It was identified in the human intracellular protein termed CD2 binding protein 2 (CD2BP2), which binds to a site containing two tandem PPPGHR segments within the cytoplasmic region of CD2. Binding experiments and mutational analyses have demonstrated the critical importance of the GYF tripeptide in ligand binding. A GYF domain is also found in several other eukaryotic proteins of unknown function []. It has been proposed that the GYF domain found in these proteins could also be involved in proline-rich sequence recognition [].Resolution of the structure of the CD2BP2 GYF domain by NMR spectroscopy revealed a compact domain with a β-β-α-β-beta topology, where the single α-helix is tilted away from the twisted, anti-parallel β-sheet. The conserved residues of the GYF domain create a contiguous patch of predominantly hydrophobic nature which forms an integral part of the ligand-binding site []. There is limited homology within the C-terminal 20-30 amino acids of various GYF domains, supporting the idea that this part of the domain is structurally but not functionally important [].This entry also matches Arabidopsis histone methyltransferases ATXR3/SDG2 and ATXR7/SDG25, which contain two partial GYF domains towards the N terminus []. Histone methyltransferase ATXR7 is involved in regulation of flowering time []. It is specifically required for the trimethylation of 'Lys-4' of histone H3 (H3K4me3) at the FLC locus, it prevents the trimethylation on 'Lys-27' (H3K27me3) at the same locus. ATXR3 is also required for H3K4 trimethylation and is crucial for both sporophyte and gametophyte development in plants [, ].
