Analysis of a 24-kilodalton protein associated with the polyhydroxyalkanoic acid granules in Alcaligenes eutrophus

A 5.0-kbp genomic EcoRI restriction fragment which complemented a third subclass of polyhydroxyalkanoic acid (PHA)-leaky mutants of A. eutrophus that accumulated PHA at a lower rate than the wild type was cloned from Alcaligenes eutrophus H16. A 687-bp phaPAe gene on this fragment encoded a 24-kDa protein (M(r) = 23,963), which was referred to as the GA24 protein. The GA24 protein was solubilized from the granules and purified to electrophoretic homogeneity, and antibodies against the GA24 protein were obtained. The GA24 protein bound to the surface of PHA granules, as revealed by immunoelectron microscopy of whole cells and of artificial PHA granules. The GA24 protein contributed approximately 5% (wt/wt) of the total cellular protein, and it was the predominant protein present in the granules. It was synthesized only in cells accumulating PHA and only in amounts that could be bound to the granules; no soluble GA24 protein was detected. Tn5::mob-induced phaPAe mutants which were unable to synthesize intact GA24 protein formed only one large PHA granule per cell. The amino acid sequence of the GA24 protein revealed two closely related stretches consisting exclusively of nonhydrophilic amino acids at the C-terminal region, which are presumably involved in the binding of GA24 to the granules, as was recently proposed for a similar protein in Rhodococcus ruber. The GA24 protein seems to be a representative of phasins, which are a new class of protein that form a layer at the surface of PHA granules, like oleosins, which form a layer at the surface of triacylglycerol inclusions in oilseed plants.