Fig. 9: An adaptive stress response induced by reduced ubiquitination and ingrained into the ubiquitin-dependent modulation of protein levels and function.

a Normally, the cargo receptor PEX5 binds in the cytosol to proteins with a PTS1 peroxisomal targeting signal type 1 and is responsible for their import into the peroxisomal matrix by binding to a cargo receptor docking complex (importomer) located on the peroxisomal membrane and composed by several other peroxins (PEXs). Mono-ubiquitination of PEX5 is known from previous studies to be mediated by the UBE2D ubiquitin-conjugating enzyme and to be key for the recycling of PEX5 back to the cytosol, where a new cycle of peroxisomal protein import can start. Consistently, UBE2D knockout has been reported to block PEX5 mono-ubiquitination and impede peroxisomal protein import. However, we find that a moderate decline in the levels of UBA1, of multiple E2s (E2 combo), and of UBE2D induces an adaptive response that paradoxically promotes peroxisomal protein import. Mechanistically, a moderate reduction in the cellular ubiquitination capacity and in UBE2D function compensates for reduced PEX5 ubiquitination by decreasing the poly-ubiquitination and proteasomal degradation of peroxins that compose the importomer complex necessary for PEX5 docking to the peroxisomal membrane. Consequently, upregulation of PEXs of the importomer complex ensures peroxisomal protein import and maintains homeostasis. These findings indicate that perturbation of E2 function can be endured via built-in adaptive mechanisms that are based on decreased ubiquitination and consequent modulation of the levels of E2 protein substrates. b In a classic stress response, stress is detected by a sensor protein, which activates an effector protein or pathway, which in turn induces an adaptive response that protects from the initial stress. c A different type of adaptive response is induced by knockdown of the E1 ubiquitin-activating enzyme UBA1, by the general but partial reduction in the function of E2 ubiquitin-conjugating enzymes (E2combo RNAi), and by knockdown of the ubiquitin-conjugating enzyme UBE2D: this “ingrained stress response” depends on the decreased turnover and consequent upregulation of UBA1/UBE2D protein substrates, as explained in (a).