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. 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 (By similarity). Extracellular and secreted via type III secretion system. Expression enhanced by contact with host cell wall. Also regulated by HrpRS and HrpL (By similarity). Belongs to the HrpA type 1 family.