Part of a signaling pathway that enables adaptation to osmotic stress through cell wall remodeling and virulence factor production. Unphosphorylated OprA forms a complex with the anti-anti-sigma-factor paralog Rv2638 that dissociates on OprA phosphorylation by PknD. Phosphorylation of OprA may stimulate the release of SigF from an inhibitory complex and enable the transcription of osmotically regulated genes, such as oprA and the ESX-1-associated virulence factor espA