Calcium binding proteins and calcium signaling in prokaryotes

•Prokaryotes maintain tight control of cytosolic calcium (Ca2+).•Several Ca2+ binding proteins have been identified in prokaryotes containing various Ca2+-binding motifs.•Knowledge on prokaryotic Ca2+ transporters is still limited.•Different aspects of Ca2+ signaling have been demonstrated, but require direct evidence. With the continued increase of genomic information and computational analyses during the recent years, the number of newly discovered calcium binding proteins (CaBPs) in prokaryotic organisms has increased dramatically. These proteins contain sequences that closely resemble a variety of eukaryotic calcium (Ca2+) binding motifs including the canonical and pseudo EF-hand motifs, Ca2+-binding β-roll, Greek key motif and a novel putative Ca2+-binding domain, called the Big domain. Prokaryotic CaBPs have been implicated in diverse cellular activities such as division, development, motility, homeostasis, stress response, secretion, transport, signaling and host-pathogen interactions. However, the majority of these proteins are hypothetical, and only few of them have been studied functionally. The finding of many diverse CaBPs in prokaryotic genomes opens an exciting area of research to explore and define the role of Ca2+ in organisms other than eukaryotes. This review presents the most recent developments in the field of CaBPs and novel advancements in the role of Ca2+ in prokaryotes.