Human tMDC III: a sperm protein with a potential role in oocyte recognition

A number of MDC (metalloproteinase-like, disintegrin-like, cysteine-rich) proteins are exclusively or abundantly expressed on mammalian sperm from a range of species, with data from rodents supporting a role for some of these in oolemma binding. However, in the human, transcripts for three of the most likely candidates have been shown to contain deletions and in-frame termination codons, rendering them non-functional. In this paper we have addressed the expression, in humans, of an additional MDC protein, tMDC III (also known as ADAM 18), previously shown to be exclusively expressed on sperm from rodents and macaques. Using a PCR-based approach, we have determined the complete nucleotide sequence of human tMDC III cDNA which, unlike the three non-functional genes, contains an uninterrupted open reading frame encoding a full-length MDC protein. Furthermore, polyclonal antisera raised against human recombinant tMDC III demonstrated the presence of mature protein on human sperm. In common with orthologues from other species, human tMDC III contains a putative integrin-binding glu-cys-asp (ECD) motif, the relevance of which is discussed in the context of a possible role in oolemma binding.