This is the second periplasmic domain (P2 domain) of the maltose transport system permease protein MalF [1-2].

1: Snapshots of the maltose transporter during ATP hydrolysis. Oldham ML, Chen J; Proc Natl Acad Sci U S A. 2011;108:15152-15156. PMID:21825153

2: Crystal structure of a catalytic intermediate of the maltose transporter. Oldham ML, Khare D, Quiocho FA, Davidson AL, Chen J; Nature. 2007;450:515-521. PMID:18033289