Possesses GTPase activator activity towards RAB32, RAB33B and RAB38 (PubMed:21808068, PubMed:26620560). Regulates the trafficking of melanogenic enzymes TYR, TYRP1 and DCT/TYRP2 to melanosomes in melanocytes by inactivating RAB32 and RAB38. Inhibits RAB32 and RAB38 activation both directly by promoting their GTPase activity and indirectly by disrupting the RAB9A-HPS4 interaction which is required for RAB32/38 activation (PubMed:26620560). . This is the function of Ensembl gene identifier ENSG00000141258 (SGSM2, small G protein signaling modulator 2).