An anti-sigma factor for extracytoplasmic function (ECF) sigma factor SigD. ECF sigma factors are held in an inactive form by an anti-sigma factor until released by regulated intramembrane proteolysis (RIP). RIP occurs when an extracytoplasmic signal triggers a concerted proteolytic cascade to transmit information and elicit cellular responses. The membrane-spanning regulatory substrate protein is first cut extracytoplasmically (site-1 protease, S1P), then within the membrane itself (site-2 protease, S2P), while cytoplasmic proteases finish degrading the regulatory protein, liberating the sigma factor (By similarity). Interacts with ECF RNA polymerase sigma factor SigD; this should inhibit the interaction of SigD with the RNA polymerase catalytic core. The cytosolic domain interacts with sigma factor SigD.