Ecm29-Dependent Proteasome Localization Regulates Cytoskeleton Remodeling at the Immune Synapse

Abstract

The formation of an immune synapse (IS) enables B cells to capture membrane-tethered antigens, where cortical actin cytoskeleton remodeling regulates cell spreading and depletion of F-actin at the centrosome promotes the recruitment of lysosomes to facilitate antigen extraction. How B cells regulate both pools of actin, remains poorly understood. We report here that decreased F-actin at the centrosome and IS relies on the distribution of the proteasome, regulated by Ecm29. Silencing Ecm29 decreases the proteasome pool associated to the centrosome of B cells and shifts its accumulation to the cell cortex and IS. Accordingly, Ecm29-silenced B cells display increased F-actin at the centrosome, impaired centrosome and lysosome repositioning to the IS and defective antigen extraction and presentation. Ecm29-silenced B cells, which accumulate higher levels of proteasome at the cell cortex, display decreased actin retrograde flow in lamellipodia and enhanced spreading responses. Our findings support a model where B the asymmetric distribution of the proteasome, mediated by Ecm29, coordinates actin dynamics at the centrosome and the IS, promoting lysosome recruitment and cell spreading.

Localized Proteasomal Degradation: From the Nucleus to Cell Periphery

“…Membrane localization of the proteasome has been documented since the early 1990′s [ 116 , 117 ]. Numerous subsequent studies have documented proteasomes in close contact with nuclear envelope-ER [ 92 , 118 , 119 , 120 , 121 , 122 , 123 , 124 ], the Golgi apparatus [ 125 , 126 ], endosomes [ 127 , 128 ], plasma membrane [ 129 , 130 ], mitochondria [ 123 , 131 , 132 , 133 , 134 , 135 , 136 ] and so on. Proteasomes at the membranes are particularly important for organelle quality control processes, such as ER-associated degradation (ERAD), endosome and Golgi-associated degradation (EGAD) and mitophagy [ 120 , 125 , 126 , 137 , 138 , 139 , 140 , 141 ].…”

Lymphocyte Polarization During Immune Synapse Assembly: Centrosomal Actin Joins the Game

“…In recent years new, unexpected players have been identified in the mechanisms that regulate centrosomal F-actin clearance and centrosome polarization during IS assembly. We and Ibañez-Vega et al have implicated the ubiquitin-proteasome system (UPS) in this process (23,58,59) (Figure 1). The UPS is a major degradation pathway in eukaryotic cells and is responsible for proteolysis of cytosolic proteins regulating a variety of cellular processes.…”

“…This pathway is exploited by both B and T lymphocytes with some cell type-dependent features. In B lymphocytes the intracellular distribution of the proteasome is regulated by the proteasome adapter and scaffold protein Ecm29 during B cell IS formation, allowing a sequential recruitment of the proteasome to the centrosome and then to the IS, which is crucial for F-actin reorganization at both locations (58,59). In T lymphocytes proteasome-mediated degradation of unknown targets at early stages of IS assembly is dependent on the transport of the 19S regulatory particle (RP) to the centrosome, which is paralleled by an active degradation of K48-linked polyubiquitylated proteins (K48 Ub) (23).…”

“…Although the proteasome is largely cytosolic, a proteasomal pool is associated with the centrosome, as witnessed by a local accumulation of proteasome subunits and proteasomal substrates around the centrosome (67). Recently, it has been reported that in B cells proteasome relocalization from the centrosome to the IS is required for the spatiotemporal coordination of centrosomal and synaptic F-actin depolymerization (58,59). In T lymphocytes a proteomic analysis of centrosomes purified from activated cells has revealed a local enrichment of proteasomal components (77).…”

The Ins and Outs of Antigen Uptake in B cells

“…Recent work provided further insight on lysosomal recruitment mechanisms. First, proteasome activity can promote actin clearance at the IS and lysosomal polarization towards immobilized antigens ( 51 , 52 ). Second, interaction with surface-tethered high-affinity antigen induces B cell permeabilization and plasma membrane repair through calcium-induced lysosome exocytosis ( 53 ).…”