Protein phosphatases come of age.

The level of phosphorylation of any protein depends on the relative activities of protein kinases and protein phosphatases (I), but the latter enzymes have received far less attention, perhaps because of the difficulty in obtaining a well defined substrate before they can be studied. Nevertheless, information about, protein serine/threonine phosphatases has increased enormously over the past few years, and readers are referred to a detailed review (2) for background information and for many of the primary references. This Minireview will concentrate on recent progress that has demonstrated key roles for these enzymes in tumor suppression and in cell division and identified novel phosphatase catalytic subunits. Our current understanding of the structure, substrate specificity, and regulation of protein phosphatases will also be discussed with emphasis on the role of “targeting subunits.” The discovery that certain membrane-bound glycoproteins are protein tyrosine phosphatases is reviewed in Ref. 3.