Microtubule-associated protein involved in membrane protein- cytoskeleton interactions. It is thought to anchor the inhibitory glycine receptor (GLYR) to subsynaptic microtubules (By similarity). Acts as a major instructive molecule at inhibitory synapses, where it also clusters GABA type A receptors (PubMed:25025157, PubMed:26613940). {ECO:0000250|UniProtKB:Q03555, ECO:0000269|PubMed:25025157, ECO:0000269|PubMed:26613940}. Also has a catalytic activity and catalyzes two steps in the biosynthesis of the molybdenum cofactor. In the first step, molybdopterin is adenylated. Subsequently, molybdate is inserted into adenylated molybdopterin and AMP is released. . This is the function of Ensembl gene identifier ENSG00000171723 (GPHN, gephyrin).