The adsorption protein of bacteriophage fd and its neighbour minor coat protein build a structural entity.

The adsorption protein g3p and another minor coat protein, g6p, are located at one end of the filamentous bacteriophage fd [Grant, R.A., Lin, T.C., Konigsberg, W.E. & Webster, R.E. (1981) J. Biol. Chem. 256, 539-546]. Both proteins, representing the proximal tip, were detached as an entity by a technique that allowed for gentle solubilization. Disrupting the phage particle with the detergent sodium deoxycholate and chloroform dissociates the major coat protein g8p, frees the phage DNA, but leaves g3p and g6p associated with each other. The g3p-g6p complex, which we termed the adsorption complex, and an oligomeric form of g3p with lower molecular mass were isolated and purified by gel-filtration chromatography in the presence of deoxycholate. These different oligomeric structures of g3p showed a different mobility in non-denaturing polyacrylamide-gel electrophoresis. Both forms were also found in non-denaturing polyacrylamide-gel electrophoresis from deoxycholate- and Triton-X-100-solubilized phage without prior chromatographic separation. The two oligomeric forms of g3p are composed of two g3p polypeptide chains in the case of the low-molecular-mass species, and four g3p and four g6p polypeptide chains for the adsorption complex.