Ubiquitination represents a critical post-translational modification of proteins,capable of indu-cing alterations in the stability,cellular localisation and activity of substrate proteins.Consequently,it plays a pivotal role in a multitude of essential cellular processes.The intracellular parasite Legionella pneumophila releases in excess of 300 effector proteins into its host cell via its distinctive type IVB secre-tion system.These effector proteins regulate the physiological activity of host cells,thereby facilitating the growth and reproduction of Legionella and ultimately resulting in Legionella infection in humans.In the context of host infection,a number of effector proteins have been identified as regulators of the host cell ubiquitination system.Together with SidJ,SdjA,DupA/DupB,LnaB,and MavL,SidEs precisely and dynamically modulate the ubiquitination pathway of host cells,thereby providing a suitable environment for L.pneumophila to survive.The clarification of the biological functions of LnaB and MavL has led to the elucidation of this complex non-canonical ubiquitination regulatory cycle in L.pneumophila.This re-view presents a summary of the structural and enzymatic basis of non-classical ubiquitination mediated by SidEs,along with an examination of its biological significance in regulating endoplasmic reticulum rear-rangement and promoting Legionella-containing vacuole formation in host cells;the mechanism by which SidJ/SdjA regulates the phosphoribosylation activity of SidEs;and DupA/DupB,LnaB and MavL reverse the ubiquitination of host substrate proteins by SidEs through a multi-step catalytic reaction.In conclu-sion,this study will provide a reference for further understanding the detailed mechanism and biological significance of this type of non-classical ubiquitination modification,as well as offering insights into the pathogenic mechanism of L.pneumophila.

Ubiquitination represents a critical post-translational modification of proteins,capable of indu-cing alterations in the stability,cellular localisation and activity of substrate proteins.Consequently,it plays a pivotal role in a multitude of essential cellular processes.The intracellular parasite Legionella pneumophila releases in excess of 300 effector proteins into its host cell via its distinctive type IVB secre-tion system.These effector proteins regulate the physiological activity of host cells,thereby facilitating the growth and reproduction of Legionella and ultimately resulting in Legionella infection in humans.In the context of host infection,a number of effector proteins have been identified as regulators of the host cell ubiquitination system.Together with SidJ,SdjA,DupA/DupB,LnaB,and MavL,SidEs precisely and dynamically modulate the ubiquitination pathway of host cells,thereby providing a suitable environment for L.pneumophila to survive.The clarification of the biological functions of LnaB and MavL has led to the elucidation of this complex non-canonical ubiquitination regulatory cycle in L.pneumophila.This re-view presents a summary of the structural and enzymatic basis of non-classical ubiquitination mediated by SidEs,along with an examination of its biological significance in regulating endoplasmic reticulum rear-rangement and promoting Legionella-containing vacuole formation in host cells;the mechanism by which SidJ/SdjA regulates the phosphoribosylation activity of SidEs;and DupA/DupB,LnaB and MavL reverse the ubiquitination of host substrate proteins by SidEs through a multi-step catalytic reaction.In conclu-sion,this study will provide a reference for further understanding the detailed mechanism and biological significance of this type of non-classical ubiquitination modification,as well as offering insights into the pathogenic mechanism of L.pneumophila.