Part of the ProU ABC transporter complex involved in glycine betaine and proline betaine uptake (PubMed:3305496, PubMed:7898450, PubMed:23249124). Probably responsible for the translocation of the substrate across the membrane (Probable). The complex is composed of two ATP-binding proteins (ProV), two transmembrane proteins (ProW) and a solute-binding protein (ProX). Belongs to the binding-protein-dependent transport system permease family. CysTW subfamily.