Novel calpain families and novel mechanisms for calpain regulation in Aplysia.

Margaret H Hastings,Xiaotang Fan,Alexander Freibauer,Katrina Gong,Caitlin Courchesne,Wayne S Sossin,Eugene A Permyakov

doi:10.1371/journal.pone.0186646

Margaret H Hastings, Xiaotang Fan + Show 5 more

Open Access

https://doi.org/10.1371/journal.pone.0186646

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Abstract

Calpains are a family of intracellular proteases defined by a conserved protease domain. In the marine mollusk Aplysia californica, calpains are important for the induction of long-term synaptic plasticity and memory, at least in part by cleaving protein kinase Cs (PKCs) into constitutively active kinases, termed protein kinase Ms (PKMs). We identify 14 genes encoding calpains in Aplysia using bioinformatics, including at least one member of each of the four major calpain families into which metazoan calpains are generally classified, as well as additional truncated and atypical calpains. Six classical calpains containing a penta-EF-hand (PEF) domain are present in Aplysia. Phylogenetic analysis determined that these six calpains come from three separate classical calpain families. One of the classical calpains in Aplysia, AplCCal1, has been implicated in plasticity. We identify three splice cassettes and an alternative transcriptional start site in AplCCal1. We characterize several of the possible isoforms of AplCCal1 in vitro, and demonstrate that AplCCal1 can cleave PKCs into PKMs in a calcium-dependent manner in vitro. We also find that AplCCal1 has a novel mechanism of auto-inactivation through N-terminal cleavage that is modulated through its alternative transcriptional start site.

Highlights

Calpains are an ancient and highly conserved superfamily of intracellular proteases with diverse roles in cellular physiology
The search yielded a protein with an incomplete catalytic domain, which we identified as a homolog of the vertebrate protein androglobin, previously known as CAPN16 or demi-calpain before its recognition as a member of the globin superfamily [49]
The calpains present in animals are classified into four conserved families defined by domains outside the catalytic domain: Small Optic Lobe (SOL) defined by N-terminal zinc fingers and a SOL domain replacing the C2-like domain III (C2L) present in all other calpain families, TRA defined by a C-terminal C2 domain, PalB defined by an N-terminal MIT domain and duplicate C2L domains, and classical calpains, defined by a C-terminal PEF domain [50]

Summary

Introduction

Calpains are an ancient and highly conserved superfamily of intracellular proteases with diverse roles in cellular physiology. Calpains are structurally diverse and many eukaryotes possess multiple distinct calpain isoforms [1]. 4 distinct calpain families are recognized based on structural differences: Classical, Small Optic Lobe (SOL), Transformer 3 (Tra) and PalB (named after the screen for acid-sensitive phosphatase mutants in the fungi, Aspergillus) families. Calpains are implicated in fundamental cellular processes including apoptosis [3] and cell division [4] and overactivation or loss of calpains is implicated in a number of pathologies [5,6,7,8].

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