Major structural protein of the Hrp pilus, which is a component of the type III secretion system (TTSS, Hrp secretion system) required for effector protein delivery, parasitism, and pathogenicity. Required for secretion of virulence proteins such as HrpW and AvrPto. The Hrp pilus functions as a conduit for protein delivery into the host cell. Also, affects the expression of TTSS-associated genes. Required for full expression of genes that encode regulatory, secretion, and effector proteins of the TTSS. HrpA-mediated gene regulation apparently is through effect on the mRNA level of hrpR and hrpS. Extracellular, secreted via type III secretion system. Expression enhanced by contact with host cell wall. Also regulated by HrpRS and HrpL. HrpA might be cleaved/degraded in 3 forms but it is not known if the observed processing has any biological significance. Analysis of mutants revealed that the C-terminus is important for pilus assembly and secretion and / or regulation function of HprA. Belongs to the HrpA type 1 family.