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ISABEL ÁLVAREZ-TABARÉS Y COLS. ANAL. REAL ACAD. NAC. FARM.

third group consists of the Asp-based protein phosphatase with a
DXDXT/V catalytic signature (10).

FIGURE 1. Classification of protein phosphatases and core configuration
of PP1. A) Schematic representation of the different families and subfamilies

of protein phosphatases. B) PP1 consists of a catalytic subunit (PP1c)
and a variable and exchangeable regulatory subunit (R).

Genomic studies have provided a catalogue of protein phospha-
tases for several eukaryotic organisms. Human DNA encodes 518
protein kinases with 428 known or predicted to phosphorylate Ser
and Thr residues and 90 belonging to the tyrosine kinase family. By
contrast, there are only 147 human protein phosphatase catalytic
subunits with 107 belonging to the Tyr phosphatase family (2, 11).
Of these 107 catalytic subunits, 81 are predicted to be active protein
phosphatases (12). Interestingly, although more than 98% of protein
phosphorylation events take place on Ser and Thr side chains, only
40 of the 147 known protein phosphatase are specific for these
aminoacids. Of these 40 phosphatases, 18 are PP2C enzymes that do
not have additional regulatory subunits. For the remaining Ser/Thr
phosphatases, function is defined by additional regulatory subunits
or interacting proteins that bind the phosphatase catalytic subunits
(and holoenzymes) target them to specific locations and/or substrates
and control their activity.

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