Fig. 2.

Schematic of STEP. There are four alternatively spliced variants of STEP (STEP₆₁, STEP₄₆, STEP₃₈, and STEP₂₀) and one calpain cleavage product (STEP₃₃). STEP₆₁ and STEP₄₆ are the major isoforms expressed in the CNS. The KIM domain is required for substrate binding, and the consensus PTP sequence, [I/V]HCxAGxxR[S/T]G, is necessary for phosphatase activity. STEP₃₈ and STEP₂₀ do not contain the PTP sequence and are inactive variants of STEP with unknown function. It is possible that these two inactive isoforms function as dominant-negative variants that compete with active STEP variants for substrate binding, or they possess other functions yet to be discovered. STEP₃₈ and STEP₂₀ contain a unique 10-amino acid sequence at their carboxyl termini that is introduced during splicing. STEP₃₃ is generated by calpain cleavage within the KIM domain between Ser²²⁴ and Leu²²⁵. Cleavage at this site disrupts the ability of STEP₃₃ to interact with its substrates. STEP₆₁ also contains an additional 172 amino acids in its amino-terminal region, which contains two transmembrane (TM) domains, two PP-rich regions, and an adjacent PEST sequence (not labeled). The TM regions target STEP₆₁ to the endoplasmic reticulum, as well as the PSD. The KIM domain is required for binding to all substrates, whereas the PP regions impart substrate specificity. PKA phosphorylates STEP within the KIM domain (Ser²²¹ and Ser⁴⁹ on STEP₆₁ and STEP₄₆, respectively), as well as in the region adjacent to the PP regions (Ser¹⁶⁰ on STEP₆₁). Although the function of Ser¹⁶⁰ phosphorylation on STEP₆₁ remains unclear, current investigations are aimed at determining whether phosphorylation at this or other sites is a signal for calpain-mediated cleavage and/or ubiquitination. Finally, two cysteine residues Cys⁶⁵ and Cys⁷⁶ are found in the TM region that mediates STEP dimerization and reduces its phosphatase activity.