THE CHEMOREPULSIVE ACTIVITY OF THE AXONAL GUIDANCE SIGNAL SEMAPHORIN-D REQUIRES DIMERIZATION

The axonal guidance signal semaphorin D is a member of a large family of proteins characterized by the presence of a highly conserved semaph orin domain of about 500 amino acids. The vertebrate semaphorins can b e divided into four different classes that contain both secreted and m embrane-bound proteins. Here we show that class III (SemD) and class I V semaphorins (SemB) form homodimers linked by intermolecular disulfid e bridges. In addition to the 95-kDa form of SemD (SemD(95k)), proteol ytic processing of SemD creates a 65-kDa isoform (SemD(65k)) that lack s the 33-kDa carboxyl-terminal domain. Although SemD(95k) formed dimer s, the removal of the carboxyl-terminal domain resulted in the dissoci ation of SemD homodimers to monomeric SemD(65k). Mutation of cysteine 723, one of four conserved cysteine residues in the 33-kDa fragment, r evealed its requirement both for the dimerization of SemD and its chem orepulsive activity. We suggest that dimerization is a general feature of semaphorins which depends on class-specific sequences and is impor tant for their function.