Recently, the Innovation Team of Agricultural Microbial Resources at the Institute of Agricultural Resources and Regional Planning (IARRP) of the Chinese Academy of Agricultural Sciences (CAAS) systematically elucidated the functional mechanism of the lytic transglycosylase, a key component of the bacterial Type III protein secretion system (T3SS). The related findings, titled "Pseudomonas syringae lytic transglycosylase HrpH interacts with host ubiquitin ligase ATL2 to modulate plant immunity," were published in the Cell Reports sub-journal of Cell.

The T3SS is an essential protein export apparatus in bacteria, consisting of approximately 20 components in a 30 kb gene cluster responsible for secreting effector proteins outside the cell and injecting them into host cells. While research on effector proteins has been quite extensive in recent years, there is still insufficient understanding of the functional mechanisms of T3SS components. In previous studies, a specific lytic transglycosylase-encoding gene, hrpH, was identified in the T3SS gene cluster. The product of this gene possesses multiple structural domains capable of breaking down bacterial cell walls and promoting effector protein secretion.

Building on this discovery, this study systematically investigated the multifunctional properties of the HrpH protein. The results revealed that HrpH not only participates in regulating effector protein transport but can also be transported to plant cells itself. The conservative lytic transglycosylase (SLT) domain and the catalytic site at position 148 glutamic acid (E148) of HrpH play crucial roles in the transport process. HrpH and SLT domain proteins in the plant apoplast can induce cell death, inhibit microbe-associated molecular pattern-induced basal immune responses (PTI), and salicylic acid (SA) signaling pathways. This function does not depend on the E148 catalytic site. Stable expression of HrpH in plants can suppress PTI responses and SA signaling pathways, reducing plant disease resistance.

Furthermore, the study found that HrpH can interact with the RING-type E3 ubiquitin ligase ATL2 in plants, with the SLT domain being a key interacting region. This interaction also does not depend on the E148 catalytic site. The research reveals the dual functions of HrpH in the transport of the Type III secretion system and its interaction with the host: as a lytic transglycosylase within bacterial cells, HrpH participates in T3SS assembly and effector protein transport; when interacting with plants, HrpH acts as an immune regulator targeting plant ubiquitin ligase to regulate plant immunity. This breakthrough challenges the traditional understanding of T3SS as a protein "injector" and provides new insights into understanding the multifunctional mechanisms of T3SS.

Assistant Researcher Li Junzhou from the Innovation Team of Agricultural Microbial Resources and Associate Professor Gu Yilin from Tianjin Agricultural University are the co-first authors. Researcher Wei Hailei from the Innovation Team of Agricultural Microbial Resources is the corresponding author, with Researcher Zhang Wei from the Department of Plant Microbiology at Cornell University as the co-corresponding author. The study was supported by various projects, including the National Key Laboratory for Efficient Utilization of Dryland and Semi-arid Farmland in Northern China, the National Natural Science Foundation, major technology projects of China Tobacco Corporation, key research and development projects of Guizhou Tobacco Corporation Zunyi City Company, the Chinese Academy of Agricultural Sciences' Science and Technology Innovation Project, and the Beijing Special Crop Innovation Team of the Modern Agricultural Industry Technology System. This research represents a new breakthrough in the ongoing efforts of Researcher Wei Hailei's team to elucidate the mechanisms of bacterial-plant host interactions.

Paper Link: https://doi.org/10.1016/j.celrep.2024.115145 

Figure 1: Interaction between HrpH and ubiquitin ligase ATL2

Figure 2: Mechanism of action of the Type III secretion system component protein HrpH