Phosphorylation of a new brain-specific septin, G-septin, by cGMP-dependent protein kinase

The septins are a family of GTPase enzymes, some of which are required for the cytokinesis stage of cell di vision and others of which are associated with exocytosis. me purified and cloned the cDNA for a 40-kDa protein from rat brain that is a substrate for type I cGMP-dependent protein kinase (PKG ). The amino acid sequences of two tryptic peptides of P40 showed high homo logy to the septins. Molecular cloning revealed the 358-amino acid P40 to b e a new member of the septin family. P40 was named G-septin, as it is phosp horylated in, vitro by PKG, but relatively poorly by the related cAMP depen dent protein kinase and not by protein kinase C, Two splice variants of G-s eptin (alpha and beta) were found with distinct N and C termini, but a comm on GTPase domain. G-septin lacks the C-terminal coiled-coil domain characte ristic of all other mammalian septins and uniquely has two predicted phosph orylation site motifs for type I PKG. Photoaffinity labeling with [alpha-P- 32]GTP confirmed that G-septin is a GTP-binding protein. Northern blotting showed that G-septin mRNA (5.0 kilobases) is highly expressed in brain and undetectable in 12 other tissues, indicating that the G-septins are primari ly neuronal proteins. Very low levels of 6.0-, 3.4-, and 2.6-kilobase trans cripts were found in testis. Our results reveal a new class of brain-specif ic septins that may be regulated by PKG in neurons.