Folding and assembly of phage P22 tailspike endorhamnosidase lacking the N‐terminal, head‐binding domain

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Abstract
Tryptic digestion of a thermal unfolding intermediate of the phage P22 tailspike endorhamnosidase produces an N‐terminally shortened protein fragment comprising amino‐acid residues 108–666 [Chen, B.‐L. & King, J. (1991) Biochemistry 30, 6260–6269]. In the present work, the 60‐kDa C‐terminal fragment was purified to homogeneity from the tryptic digest by gel‐fitration chromatography. As is the case for the whole tailspike protein (72 kDa), the purified fragment was found to remain stably folded as a highly soluble, SDS‐resistant, enzymatically active trimer. However, its unfolding in the presence of guanidinium chloride was accelerated at least 10‐fold compared to the complete, native tailspike protein. Shortened tailspike trimers reconstituted spontaneously and with high yield after diluting a solution containing acid‐urea‐unfolded fragment polypeptides with neutral buffer. Upon recombinant expression of the 60‐kDa polypeptide in Escherichia coli, it also assembled efficiently and formed SDS‐resistant trimers. The refolding and assembly pathway of the N‐terminally shortened tailspike paralleled that of the complete protein with slightly, but significantly, accelerated folding reactions, at both the subunit and the trimer levels. As found for the complete tailspike protein, yields of refolding and assembly of the 60‐kDa fragments into SDS‐resistant trimers decreased with increasing temperature. The refolding yield of fragments derived from a temperature‐sensitive mutant (Gly244 → Arg) tailspike protein was affected in similar fashion as shown for the whole protein. We conclude that the N‐terminal domain (residues 1–107) is dispensable for folding and assembly of the P22 tailspike endorhamnosidase both in vitro and in vivo.