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. Belongs to the HrpA type 1 family.