This is the non-catalytic subunit of the N-terminal acetyltransferase B complex (NatB). The NatB complex catalyses the acetylation of the amino-terminal methionine residue of all proteins beginning with Met-Asp or Met-Glu and of some proteins beginning with Met-Asn or Met-Met. In Saccharomyces cerevisiae (Baker's yeast) this subunit is called MDM20 and in Schizosaccharomyces pombe (Fission yeast) it is called Arm1. NatB acetylates the Tpm1 protein and regulates and tropomyocin-actin interactions. This subunit is required by the NatB complex for the N-terminal acetylation of Tpm1 [].
Tropomyosins [], are a family of closely related proteins present in muscle and non-muscle cells. In striated muscle, tropomyosin mediate the interactions between the troponin complex and actin so as to regulate muscle contraction []. The role of tropomyosin in smooth muscle and non-muscle tissues is not clear. Tropomyosin is an α-helical protein that forms a coiled-coil structure of 2 parallel helices containing 2 sets of 7 alternating actin binding sites []. There are multiple cell-specific isoforms, created by differential splicing of the messenger RNA from one gene, but the proportions of the isoforms vary between different cell types. Muscle isoforms of tropomyosin are characterised by having 284 amino acid residues and a highly conserved N-terminal region, whereas non-muscle forms are generally smaller and are heterogeneous in their N-terminal region.This entry represents tropomyosin (Tmp) 1, 2 and 3. Within the yeast Tmp1 and Tmp2, biochemical and sequence analyses indicate that Tpm2 spans four actin monomers along a filament, whereas Tpm1 spans five. Despite its shorter length, Tpm2 can compete with Tpm1 for binding to F-actin. Over-expression of Tpm2 in vivo alters the axial budding of haploids to a bipolar pattern, and this can be partially suppressed by co-over-expression of Tpm1. This suggests distinct functions for the two tropomyosins, and indicates that the ratio between them is important for correct morphogenesis [].
