Serine/threonine protein phosphatases.

1 (1-1) and inhibitor-2 (1-2), and preferentially dephosphorylate the /3-subunit of phosphorylase kinase, whereas type-2 phosphatases are insensitive to the heat-stable inhibitors and preferentially dephosphorylate the x-subunit of phosphorylase kinase [1,2]. Type-2 phosphatases can be further subdivided into spontaneously active (PP2A), Ca2l-dependent (PP2B) and Mg2+dependent (PP2C) classes. The use of okadaic acid, a specific phosphatase inhibitor, further facilitated the discrimination between different classes [3]. Although still widely in use, this classificatioh does not reflect the actual phylogenetic relationship between the different Ser/Thr protein phosphatases. Molecular cloning revealed that PP2A was in fact much more closely related to PPI than to PP2C. Moreover, in the past few years many novel protein phosphatases have been identified that do not fit into the above classification. Many of these protein phosphatases are closely related to the existing classes or are intermediates between classes. From a phylogenetic point of view, it would be more reasonable to group PP 1, PP2A and PP2B in a family I and PP2C in a family II. Recently a detailed comparison of the primary structures of 44 different protein Ser/Thr phosphatases, excluding PP2C, was carried out [4]. This revealed a common core structure that comprises two domains. The first domain is predicted to fold as