Participation of protein sequence termini in crystal contacts.

The analysis of the crystal packing interactions, in a nonredundant set of high resolution and monomeric globular protein crystal structures, shows that the residues located at the N- and C-termini of the sequence tend to participate in packing interaction more often than expected and that often they interact with each other. Since the sequence termini are, in general, conformationally very flexible and since they host electrical charges of opposite sign, it can be hypothesized that they play a crucial role in the early formation of the nonphysiological contacts that bring to protein crystallization. It is thus not surprising that modest lengthening/shortening of the sequence termini have often a dramatic effect on protein crystallogenesis.