1.B.63 The Imipenum resistance-associated porin, CarO (CarO) Family Resistance to both imipenem and meropenem in multidrug-resistant clinical strains of Acinetobacter baumannii is associated with the loss of a heat-modifiable 25/29-kDa outer membrane protein called CarO, a channel-forming protein. Mass spectrometry analyses of this protein band detected another 25-kDa protein (called Omp25; TC# 1.B.1.11.1) with CarO. Both proteins presented similar physicochemical parameters (MW and pI). Siroy et al. (2005) overproduced and purified the two polypeptides as His-tagged recombinant proteins. Circular dichroism analyses demonstrated that the secondary structure of these proteins consist mainly of beta-strands with spectra typical of porins. They studied the channel-forming properties of both proteins by reconstitution into artificial lipid bilayers. CarO induced ion channels with a conductance value of 110 pS in 1 M KCl, but the Omp25 protein did not form channels despite its suggested porin function. The pores formed by CarO showed slight cation selectivity and no voltage closure. No specific imipenem binding site was found in CarO which may form non-specific monomeric channels.
References:
AlQumaizi, K.I., S. Kumar, R. Anwer, and S. Mustafa. (2022). Differential Gene Expression of Efflux Pumps and Porins in Clinical Isolates of MDR. Life (Basel) 12:.
Fonseca, E.L., E. Scheidegger, F.S. Freitas, R. Cipriano, and A.C. Vicente. (2013). Carbapenem-resistant Acinetobacter baumannii from Brazil: role of carO alleles expression and blaOXA-23 gene. BMC Microbiol 13: 245.
Siroy, A., V. Molle, C. Lemaître-Guillier, D. Vallenet, M. Pestel-Caron, A.J. Cozzone, T. Jouenne, and E. Dé. (2005). Channel formation by CarO, the carbapenem resistance-associated outer membrane protein of Acinetobacter baumannii. Antimicrob. Agents Chemother. 49: 4876-4883.
Yang, H., L. Huang, P.A. Barnie, Z. Su, Z. Mi, J. Chen, V. Aparna, D. Kumar, and H. Xu. (2015). Characterization and distribution of drug resistance associated β-lactamase, membrane porin and efflux pump genes in MDR A. baumannii isolated from Zhenjiang, China. Int J Clin Exp Med 8: 15393-15402.
Zahn, M., T. D''Agostino, E. Eren, A. Baslé, M. Ceccarelli, and B. van den Berg. (2015). Small-Molecule Transport by CarO, an Abundant Eight-Stranded β-Barrel Outer Membrane Protein from Acinetobacter baumannii. J. Mol. Biol. 427: 2329-2339.
Zhu, L.J., X.Y. Chen, and P.F. Hou. (2019). Mutation of CarO participates in drug resistance in imipenem-resistant Acinetobacter baumannii. J Clin Lab Anal 33: e22976.
Examples:
TC#	Name	Organismal Type	Example
1.B.63.1.1	The CarO porin is slightly cation-selective and can be mutated to give rise to imipenem-resistance (Zhu et al. 2019). It is of 243 aas (Siroy et al. 2005) and plays a role in cabapenum resistance (Fonseca et al. 2013) as well as MDR (Yang et al. 2015). It has been implicated in the uptake of ornithine as well as carbapenem antibiotics. Zahn et al. 2015 reported crystal structures of three isoforms of CarO. The structures show a monomeric eight-stranded barrel lacking an open channel. CarO has a substantial extracellular domain resembling a glove that contains all the divergent residues between the different isoforms. A6XB80 is another isoform with 77% identity to the one listed here in TCDB. Overexpression of carO is associated with carbapenem resistance (AlQumaizi et al. 2022).	Proteobacteria	CarO of Acinetobacter baumannii

1.B.63.1.2	CarO homologue of 310 aas	Cyanobacteria	CarO homologue of Oscillatoria acuminata

1.B.63.1.3	CarO homologue of 275 aas	Acidobacteria	CarO homologue of Acidobacterium capsulatum

1.B.63.1.4	CarO homologue of 223 aas	Proteobacteria	CarO homolgoue of Hirschia baltica

1.B.63.1.5	CarO homolgue of 233 aas	Bacteroidetes	CarO homologue of Sprosoma linguale

1.B.63.1.6	CarO homologue of 273 aas.	Bacteroidetes	CarO homologue of Porphyromonas gingivalis