Polyphosphate Granules and Acidocalcisomes

Abstract

Polyphosphate (polyP) is a linear polymer of phosphate residues linked by energy-rich phospho-anhydride bonds. It can be formed abiotically by heating of phosphate solutions (e.g. by vulcanism) or biologically by the action of enzymes, so-called polyphosphate kinases (PPKs). PolyP is an ubiquitous polymer present in virtually all organisms that have been looked at and therefore should be of fundamental importance. It can have an astonishing variety of functions in different species from that of a reservoir for phosphorous in many prokaryotes to a part of the blood coagulation system in mammalians. This chapter focuses only on polyP in prokaryotes where it is present in the form of granule-like inclusions of roughly 50 to about 500 nm diameter. Despite their ubiquitous occurrence in bacteria, the structures and composition of polyP granules in bacteria are only poorly understood. In this chapter I will provide an update of the various functions polyP can have in prokaryotes, will give an overview on PPK enzymes and describe some recent findings showing that polyP granules have an astonishing high number of proteins specifically bound to the surface of polyP granules (polyphosphatosomes). In addition, I will discuss the pros and cons whether acidic, membrane-enclosed, polyP and calcium-ions containing organelles (acidocalcisomes) exist in prokaryotes.