SppA peptidases: family diversity from heterotrophic bacteria to photoautotrophic eukaryotes

Serine-type SppA peptidases are found in viruses, archaea, eubacteria and in chloroplasts. The family consists of two homologous groups of proteins, designated SppA1 and SppA2, which differ in molecular weight and domain organization. SppA1 is a high molecular weight protein of 70 kDa with two homologous domains with protease signatures. SppA2 represents a half-size SppA1 comprised of only one protease domain. The genomes of most heterotrophic and photosynthetic bacteria encode both, SppA1 and SppA2 proteins, while genome of higher plants, Arabidopsis and rice, encode only one protease, SppA1. An intriguing feature of SppA proteases is their similarity to ClpP proteases in catalytically active regions. SppA functions and gene expression differ between heterotrophic and photosynthetic organisms, in which SppA expression is light-regulated. Apparently, SppA regulation and substrate specificities were modified in photosynthetic organisms and are essential for acclimation of the thylakoid membrane system to light.