Figure 2.

Structure of the P Pilus Rod

(A) Surface diagram of the rod. Each subunit is shown in surface representation, color coded differently. The rod is oriented in such a way that the N termini of each subunit (the staples) are directed toward the top. In that orientation, the OM and tip fibrillum are toward the bottom and top, respectively. The subunit in cyan is the reference subunit and is numbered “0.” Subunits above or below this subunit are assembled before or after subunit 0, respectively, and are therefore numbered negatively (−1 to −6) or positively (+1 to +6), respectively.

(B) Surface diagram focusing on the Ntes. The orientation of the pilus rod structure and the colorcoding of subunits are the same as in (A), but only the Ntes are shown, clearly illustrating the ascending path that the Ntes form within the structure. The rise from one subunit to another is indicated.

(C) Top view of the pilus rod. The rod is represented as in (A). The Nte of the last subunit (−6) has been removed for clarity.

(D) Ribbon diagram of the structure of PapA in the rod (subunit 0) in donor-strand exchange with the subunit next in assembly (subunit +1). The subunit is shown in cyan (labeled “PapA _{subunit 0}”) with the Nte of subunit +1 colored in orange (labeled “PapA Nte _subunit +1”). Secondary structure elements are labeled. The orientation of the subunit is the same as in (A). In that orientation, the staple extends approximately parallel to the pilus axis (indicated by an arrow).