ESSENTIAL ROLE OF A SODIUM DODECYL SULFATE-RESISTANT PROTEIN-IV MULTIMER IN ASSEMBLY-EXPORT OF FILAMENTOUS PHAGE

Filamentous phage f1 encodes protein IV (pIV), a protein essential for phage morphogenesis that localizes to the outer membrane of Escherich ia coil, where it is found as a multimer of 10 to 12 subunits. Introdu ction of internal His or Strep affinity tags at different sites in pIV interfered with its function to a variable extent. A spontaneous seco nd-site suppressor mutation in gene IV allowed several different inser tion mutants to function. The identical mutation was also isolated as a suppressor of a multimerization-defective missense mutation. A high- molecular-mass pIV species is the predominant form of pIV present in c ells. This species is stable in 4% sodium dodecyl sulfate at temperatu res up to 65 degrees C and is largely preserved at 100 degrees C in La emmli protein sample buffer containing 4% sodium dodecyl sulfate, The suppressor mutation makes the high-molecular-mass form of wild-type pI V extremely resistant to dissociation, and it stabilizes the high-mole cular-mass form of several mutant pIV proteins to extents that correla te with their level of function. Mixed multimers of pIV(f1) and pIV(Ik e) also remain associated during heating in sodium dodecyl sulfate-con taining buffers, Thus, sodium dodecyl sulfate- and heat-resistant high -molecular-mass pIV is derived from pIV multimer and reflects the phys iologically relevant form of the protein essential for assembly-export .