Arabidopsis PPP family of serine/threonine protein phosphatases: many targets but few engines

• PPP family serine/threonine phosphatase catalytic subunits are few in number compared to the protein kinases of plants.
• PPP family phosphatase catalytic subunits display promiscuity with respect to substrates but obtain specificity (and numbers) through many additional regulatory subunits.
• The PPP enzymes are key regulators of many signaling, metabolic, and developmental pathways in plants.
• Arabidopsis PP1 binds regulatory proteins via an RVxF motif.

The major plant serine/threonine protein phosphatases belong to the phosphoprotein phosphatase (PPP) family. Over the past few years the complement of Arabidopsis thaliana PPP family of catalytic subunits has been cataloged and many regulatory subunits identified. Specific roles for PPPs have been characterized, including roles in auxin and brassinosteroid signaling, in phototropism, in regulating the target of rapamycin pathway, and in cell stress responses. In this review, we provide a framework for understanding the PPP family by exploring the fundamental role of the phosphatase regulatory subunits that drive catalytic engine specificity. Although there are fewer plant protein phosphatases compared with their protein kinase partners, their function is now recognized to be as dynamic and as regulated as that of protein kinases.