Auxiliary transport proteins. Proteins that function with or are complexed to known transport proteins are included in this category. An example would be the membrane fusion proteins that facilitate transport across the two membranes of the Gram-negative bacterial cell envelope in a single step driven by the energy source (ATP or the pmf) utilized by a cytoplasmic membrane transporter. Energy coupling and regulatory proteins that do not actually participate in transport represent other possible examples. In some cases auxiliary proteins are considered to be part of the transport system with which they function, and in such cases no distinct entry in category 8 is provided.
8.A Auxiliary transport proteins. Proteins that in some way facilitate transport across one or more biological membranes but do not themselves participate directly in transport are included in this class. These proteins always function in conjunction with one or more established transport systems. They may provide a function connected with energy coupling to transport, play a structural role in complex formation, serve a biogenic or stability function or function in regulation.