A symbiont-produced protein and bacterial symbiosis in Amoeba proteus.

Gram symbiotic X-bacteria present in the xD strain of Amoeba proteus as required cell components, synthesize and export a large amount of a 29-kDa protein (S29x) into the host's cytoplasm across bacterial and symbiosome membranes. The S29x protein produced by E. coli transformed with the s29x gene is also rapidly secreted into the culture medium. Inside amoebae, S29x enters the host's nucleus as detected by confocal and immunoelectron microscopy, although it is not clear if S29x is selectively accumulated inside the nucleus. The deduced amino-acid sequence of S29x has a stretch of basic amino acids that could act as a nuclear localization signal, but there is no signal peptide at the N-terminus and the transport of S29x is energy independent. The functions of S29x are not known, but in view of its prominent presence inside the amoeba's nucleus, S29x is suspected to be involved in affecting the expression of amoeba's nuclear gene(s).