Found in the main part of the sperm flagellum, the fibrous sheath is a cytoskeletal structure comprising two longitudinal columns connected by closely spaced circumferential ribs []. A-kinase anchoring proteins (AKAPs) secure cyclic AMP-dependent protein kinases within specific cytoplasmic domains; most abundant in the fibrous sheath is AKAP4 []. AKAP4 has been shown to bind AKAP3 and two spermatogenic cell-specific proteins, Fibrous Sheath Interacting Proteins 1 and 2 (FSIP1, FSIP2) [].How the fibrous sheath assembles is not yet fully understood. AKAP4 is synthesised and incorporated into the nascent fibrous sheath late in spermatid development; its precursor is processed in the flagellum, and only the mature form of AKAP4 appears to bind AKAP3 []. It is likely, therefore, that AKAP3 is involved in organising the basic structure of the fibrous sheath, while AKAP4 helps to complete fibrous sheath assembly.The FSIP1 protein is 435 amino acids in length, and is rich in glutamine and asparagine []. Minimum domains for its binding to AKAP4 have been localised to amino acids 351-359 and 692-721 of AKAP4 constructs [].
