P311, also called PTZ17, was identified by suppressive subtraction hybridization as potentially involved in smooth muscle (SM) myogenesis., an 8-kDa polypeptide, was previously shown to be highly expressed in invasive glioma cells. P311 is constitutively serine-phosphorylated; decreased phosphorylation is observed in migration-activated glioma cells. The primary amino acid sequence of P311 indicates a putative serine phosphorylation site (S59) near the PEST domain. Site-directed mutagenesis of S59A retarded P311 degradation and induced glioma cell motility. In contrast, S59D mutation resulted in the rapid degradation of P311 and reduced glioma cell migration.