Semenogelin I: Proteins and Enzymes

Semenogelin I (SgI) and Semenogelin II (SgII) are the major seminal vesicle secreted proteins in human semen (reviewed in Lundwall, 2002 and Bonilha et al, 2006). SgI and SgII, in semen both originate from the glandular epithelium of the seminal vesicles which secrete them at high concentrations. SgII is also secreted by the epididymis at lower concentrations. SgI and SgII interact both non-covalently and covalently by disulphide bridges to instantly form a gel-like coagulum upon ejaculation. Fibronectin is also a key component of the coagulum. The gel structure dissolves spontaneously within minutes after ejaculation as a result of proteolytic degradation of SgI/SgII by prostatic serine protease (PSA) and other proteases which cleave SgI/SgII into fragments. The high concentration of SgI/SgII in human semen led to the concept of SgI/SgII as a potentially useful marker for semen identification (reviewed in Pang and Cheung, 2006). Whereas SgI/SgII were originally identified as the major components present in seminal vesicle secretion, they are now known to be expressed in a number of tissues. The list includes seminal vesicles, epididymis, vas deferens, prostate, skeletal muscle, kidney, colon, trachea, lung, breast and retina as well as in several malignant tissues and cell lines (reviewed in Lundwall, 2002 and Bonilha et al, 2006). The non-genital expression of SgI/SgII suggests that these proteins also have functions that are unrelated to sperm. Human Sg1 is a non-glycosylated protein of 439 amino acids with a molecular weight of approx. 50 kDa. A few percent of the worlds population also carry an allele that gives rise to a truncated SgI molecule with a molecular mass of 43 kDa. Human SgII is a 559 amino acid protein with a molecular weight of approx. 63 kDa. SgII has a potential site for N-linked glycosylation and approximately half of the molecules in human semen are glycosylated yielding two SgII species with a different of 5 kDa. SgI has a single Cys residue and SgII has two Cys residues, and the molecules can exist as covalent homo- and heteromultimers.