Small heat shock proteins: recent developments

Small heat shock proteins (sHSPs) are abundantly present in many different organisms at elevated temperatures. Members of the subgroup of alpha crystallin domain (ACD)-type sHSPs belong to the large family of protein chaperones. They bind non-native proteins in an ATP-independent manner, thereby holding the incorporated clients soluble for subsequent refolding by other molecular chaperoning systems. sHSPs do not actively refold incorporated peptides therefore they are sometimes referred to as holdases. Varying numbers of sHSPs have been documented in the different domains of life and dependent on the analyzed organism. Generally, diverse sHSPs possess more sequence similarities in the conserved ACD, whereas the N- and C-terminal extensions are less conserved. Despite their designation as sHSPs, they are not solely present during heat stress. sHSPs presumably help to protect cells under various stresses, but they were also found during development, e.g., in embryonic development of higher plants which is associated with ongoing seed desiccation. The functional and physiological relevance of several different sHSPs in one organism remains still unclear, especially in plants where several highly similar sHSPs are present in the same compartment. The wide range of biotic and abiotic stresses that induce the expression of multiple sHSP genes makes it challenging to define the physiological relevance of each of these versatile proteins.