p32 protein levels are integral to mitochondrial and endoplasmic reticulum morphology, cell metabolism and survival

Abstract

p32 [also known as HABP1 (hyaluronan-binding protein 1), gC1qR (receptor for globular head domains complement 1q) or C1qbp (complement 1q-binding protein)] has been shown previously to have both mitochondrial and non-mitochondrial localization and functions. In the present study, we show for the first time that endogenous p32 protein is a mitochondrial protein in HeLa cells under control and stress conditions. In defining the impact of altering p32 levels in these cells, we demonstrate that the overexpression of p32 increased mitochondrial fibrils. Conversely, siRNA-mediated p32 knockdown enhanced mitochondrial fragmentation accompanied by a loss of detectable levels of the mitochondrial fusion mediator proteins Mfn (mitofusin) 1 and Mfn2. More detailed ultrastructure analysis by transmission electron microscopy revealed aberrant mitochondrial structures with less and/or fragmented cristae and reduced mitochondrial matrix density as well as more punctate ER (endoplasmic reticulum) with noticeable dissociation of their ribosomes. The analysis of mitochondrial bioenergetics showed significantly reduced capacities in basal respiration and oxidative ATP turnover following p32 depletion. Furthermore, siRNA-mediated p32 knockdown resulted in differential stress-dependent effects on cell death, with enhanced cell death observed in the presence of hyperosmotic stress or cisplatin treatment, but decreased cell death in the presence of arsenite. Taken together, our studies highlight the critical contributions of the p32 protein to the morphology of mitochondria and ER under normal cellular conditions, as well as important roles of the p32 protein in cellular metabolism and various stress responses.

Arginase II Contributes to the Ca 2+ /CaMKII/eNOS Axis by Regulating Ca 2+ Concentration Between the Cytosol and Mitochondria in a p32‐Dependent Manner

“…These results suggest that [p32]c can be released from mitochondrial p32 during mitochondrial events. p32 silencing enhanced mitochondrial fission and the loss of mitochondrial fusion with the loss of detectable levels of Mfn1/2 proteins 34. Additionally, the difference in molecular weight of p32 observed in Western blot and MALDI‐TOF mass spectrometry analysis is probably because of glycosylation and strong charges 35…”

Novel roles for actin in mitochondrial fission

“…One goal of our model is to highlight the relevant sizes of the key players, because the dimensions of the ER, mitochondrion and fission apparatus put constraints on the possible mechanisms. For instance, the narrow widths of mammalian mitochondria from many cell types, with diameters frequently in the range of only 150-300 nm, restrict the possible ways in which force can be applied by actin and myosin (Goldstein et al, 1984;Hu et al, 2013;Jans et al, 2013;Kim et al, 2012;Noske et al, 2008;Perkins and Ellisman, 2011;Vafai and Mootha, 2012). In addition, we present the possibility that there might be multiple mechanisms by which actin participates in fission.…”

“…One morphological aspect of direct relevance to fission is mitochondrial diameter. Diameters range between 150 and 300 nm in a variety of cells (Goldstein et al, 1984;Hu et al, 2013;Jans et al, 2013;Kim et al, 2012;Noske et al, 2008;Perkins and Ellisman, 2011;Vafai and Mootha, 2012), with a narrow range of variation within a single cell type. These diameters are far narrower than the 500-1000 nm estimates given in some textbooks and reviews.…”

Reactivation of latently infected HIV-1 viral reservoirs and correction of aberrant alternative splicing in the LMNA gene via AMPK activation: Common mechanism of action linking HIV-1 latency and Hutchinson–Gilford progeria syndrome

“…p32, mitochondria, ASF/SF2, and alternative splicing Originally copurified with the splicing factor ASF/SF2, p32, also known as gC1qR or HABP1, has subsequently been shown to be primarily localized to the mitochondrial matrix, potentially functioning as a protein chaperone facilitating mitochondrial DNA transcription and translation as well as maintaining the efficiency of oxidative phosphorylation [47][48][49][50]. p32 has been shown to associate with mitochondrial transcription factor A (TFAM) and abrogation of p32 expression leads to a reduction in mitochondrial matrix density, basal respiration, oxidative ATP turnover, cristae fragmentation, and a reduction of the mitochondrial fusion proteins mitofusion (Mfn) 1 and Mfn2 [51,52]. p32 also interacts with all mitochondrial messenger RNA species and is likely indispensible for mitochondrial translation and protein synthesis, as evidenced by a reduction in the levels of electron transport chain complexes I, III, IV, and V and inhibition of oxidation of NADH and cytochrome c on cellular knockdown of p32 [53,54].…”