A novel 50 kDa protein forms complexes with protein phosphatase 4 and is located at centrosomal microtubule organizing centres.

Protein phosphatase 4 (PPP4) is a protein serine/threonine phosphatase that has been implicated in microtubule organization at centrosomes. Complexes of PPP4 with high apparent molecular masses (450 and 600 kDa) were purified from mammalian skeletal muscle and testis to near homogeneity. Amino acid sequences derived from a protein component present in both complexes were utilized to identify a human cDNA. The encoded putative PPP4 regulatory subunit (termed PPP4R2), comprising 453 amino acids, had a molecular mass of 50.4 kDa. The interaction of PPP4R2 with PPP4 catalytic subunit (PPP4c) was confirmed by co-sedimentation of PPP4c with PPP4R2 expressed in bacteria and human cells. PPP4c formed a complex of 450 kDa with baculovirus expressed His(6)-tagged PPP4R2. Immunocytological detection of PPP4R2 at centrosomes suggests that it may target PPP4c to this location. Native 450 kDa and 600 kDa PPP4 complexes are inactive, but can be activated by basic proteins, suggesting that PPP4R2 may also regulate the activity of PPP4c at centrosomal microtubule organising centres.