Possible positive regulator of sigma-B activity (By similarity). Non-phosphorylated BtrV binds to BtrW, preventing its association with an unknown partner(s) that might be sigma-B. When phosphorylated, releases BtrW, which is then free to complex with and inactivate its partner. Involved in type III secretion system (TTSS). Interacts with BtrW. Phosphorylated by BtrW. Dephosphorylated by BtrU. Type III secreted proteins Bsp22 and BopD accumulate intracellularly instead of being secreted in cells expressing the mutagenized BtrV. Belongs to the anti-sigma-factor antagonist family.