9.A.18 The Peptide Uptake Permease (PUP) Family

Two proteins, the SbmA protein (406 aas) of E. coli and the BacA protein (420 aas) of Rhizobium meliloti comprise the PUP family. SbmA has been reported to be the permease for uptake of thiazole ring-containing peptide antibiotics such as Microcin B17 and Microcin J25 as well as the non-peptide antibiotic, bleomycin, across the cytoplasmic membrane (Salomón and Farías, 1995), and BacA is a nodulation protein essential for bacterial development when Rhizobium is in symbiosis with a leguminous plant such as alfalfa (Glazebrook et al., 1993). These two proteins exhibit 64% identity and are functionally interchangeable in both E. coli and R. meliloti (Ichige et al., 1997).

R. meliloti bacA null mutants show increased resistance to bleomycin and certain aminoglycosides as well as increased sensitivity to ethanol and detergents. The latter properties are not characteristic of E. coli sbmA mutants. It has been hypothesized that BacA may take up peptide substances required for developmental progression towards bacteroid formation. BacA (but not SbmA) may also play a role in the maintenance of membrane integrity (see below).

PUP family proteins are homologous to, but show a low degree of sequence similarity to, a few putative ABC-type transporters in Gram-negative and Gram-positive bacteria which, unlike SbmA and BacA, possess ATP-binding cassette (ABC)-containing domains. SbmA and BacA also differ from these putative ABC proteins in possessing 7 rather than 6 putative transmembrane α-helical spanners. It is possible that ABC-protein constituents of the PUP family will be found, but the mechanism of energy coupling to PUP family permeases is not currently known. A BacA homologue in Mycobacterium tuberculosis with an ATP hydrolyzing domain takes up antimicrobial peptides and bleomycin, (Q50614; TC#3.A.1.203.4; Domenech et al. 2009). It is possible that all BacA homologues will prove to belong to the ABC superfamily, but the cytoplasmic ATPase domains of the Rhizobial and Escherichia homologues have not been identifited.

Ferguson et al., 2004 reported (1) that BacA of Sinhorizobium and Brucella shows sequence similarity to the long-chain fatty acid transporter ABCD1 (ALD, the adrenoleukodystrophy protein; TC #3.A.1.203.3), (2) that bacA mutants show increased sensitivity to detergents and other cell disrupting agents, and (3) that BacA affects the very long-chain fatty acid (27-hydroxy-C28:0 and 29-hydroxy-C30:0 fatty acids) content of the lipid A constituent of lipopolysaccharides. This led to the postulate that BacA is an export permease for activated long-chain fatty acids, required for the synthesis of lipid A (Ferguson et al., 2004). Direct transport and mode of energy coupling have yet to be demonstrated. However, Ardissone et al. (2011) showed that BacA of a Rhizobium species is a peptide uniporter essential for bacteroid differentiation.

The generalized reactions currently proposed for proteins of the PUP family are:

(1) Peptide antibiotic (periplasm) →  Peptide antibiotic (cytoplasm)

(2) Activated long-chain fatty acid (cytoplasm)  →  Activated long-chain fatty acid (periplasm)