HSP40: Localization

In most cases, DNAJs/HSP40s are homodimeric proteins residing in the cytosol of bacteria and certain Archaea ^{9, 100} and the cytosol ⁸, nuclei ⁸, endosomes ⁹⁵, exosomes ⁷¹, mitochondria ¹⁷⁷, ribosomes ⁸, chloroplasts ¹⁷⁸, and ER ¹⁷⁹ of eukaryotes. In humans, seven types of ER-resident DNAJs/HSP40s have been found (DnaJC1/ERdj1/Mtj1, DnaJC23/ERdj2/Sec63, DnaJB11/ERdj3/HEDJ, DnaJB9/ERdj4/Mdg1, DnaJC10/ERdj5/JPD1, DnaJC3/ERdj6/p58, DnaJB14/EGNR9427), while yeast expresses five DNAJs/HSP40s in the ER (Scj1, Jem1, Hlj1, Sec63, Erj5) ¹⁰. Amongst them, Scj1 and Jem1 are crucially involved in protein folding and quality control in the ER, whereas Sec63 functions in protein translocation across the ER membrane as partner for the ER-resident HSP70, BiP. A total of six types of ER-resident DNAJs/HSP40s have been found in plants (p58A^IPK, p58B^IPK , ERdj2A, ERdj3A, ERdj3B, and ERdj7) ^{103, 104}. While Arabidopsis has two copies of ERDJ2 (ERDJ2A and ERDJ2B) ¹⁰³, rice bears one copy ¹⁰⁴. Rice has two P58IPK gene copies (OSP58A and OSP58B) ¹⁰⁴, whereas Arabidopsis has only one copy ¹⁰³. Expression of AtERdj2B is induced by ER stress, whereas AtERdj2A is constitutively expressed ¹⁰³. A similar situation is found in rice where Osp58A^IPK is constitutively expressed while Osp58B^IPK is induced by ER stress ¹⁰⁴. Thus, the duplicated genes encoding ERdjs might play distinct roles in different plants ¹⁰⁴.

DNAJs/HSP40s can be expressed tissue-specifically or universally in all tissues in multicellular organisms. For instance, Northern blot analyses revealed that human brain expresses DnaJB2/Hsj1 to a much higher degree than other tissues examined, and hybridization studies with riboprobes in situ show a restricted pattern of expression of the mRNA within the brain ⁸⁰. Yang and co-workers recently identified a novel member of the DNAJ/HSP40 family in rats, rDJL that is preferentially expressed in testis and is located primarily to the acrosome region of spermatozoa ¹⁸⁰. The tissue-specific expression can be achieved by alternative splicing of the corresponding DNAJ/HSP40 mRNA. In this regard, two transcript variants encoding DnaJA1/Hdj2 have been identified for the DNAJA1 gene leading to the expression of two DnaJA1/Hdj2 isoforms differing from each other by 95 amino acids ⁶⁹. While one of the alternatively spliced isoforms of DnaJA1/Hdj2 was found to reside in the nucleus, indicating that it acts as a nuclear co-chaperone, the other isoform was observed throughout the cell most likely due to the elimination of a putative nuclear localization signal sequence ⁶⁹. Moreover, isoform 1 of DnaJA1/Hdj2 was found as being highly expressed in brain and other tissues while the other alternatively spliced isoform was highly abundant in testis and sperm ⁷⁰.

In addition to their intracellular location, DNAJs/HSP40s have been found in the extracellular milieu. How DNAJs/HSP40s are found in the extracellular medium remains enigmatic as HSPs in general lack the consensus signal for secretion via the classical Golgi pathway. The mechanisms underlying its export involve, on the one hand, an active secretory process with the participation of exosomes. Gonzales et al. detected DnaJA1, DnaJA2, DnaJB1, DnaJC7, and DnaJC13 in urinary exosomes from normal human subjects ⁷¹. Urinary exosomes are small extracellular vesicles (<100 nm in diameter) that originate from the internal vesicles of multivesicular bodies (MVB) in renal epithelial cells, such as glomerular podocytes, renal tubule cells, and the cells lining the urinary drainage system ¹⁸¹. Exosome release into the urine occurs after fusion of the outer membrane of the MVB with the apical plasma membrane of epithelial cells.

On the other hand, ER-resident DnaJB11/ERdj3 could be identified most recently in the extracellular space upon activation of the unfolded protein response (UPR) which is known to impact extracellular proteostasis through transcriptional remodeling of the ER proteostasis pathways ¹⁸². Such remodeling processes downregulate the release of misfolded, aggregation-prone proteins during ER stress. Genereux and collaborators provide evidence for the UPR-induced upregulation and secretion of DnaJB11/ERdj3 into the extracellular environment thereby enhancing the extracellular proteostasis capacity ¹⁸². Remarkably, DnaJB11/ERdj3 was found as being co-secreted in a stable complex with misfolding-prone protein clients thus connecting intracellular and extracellular proteostasis capacity during ER stress ¹⁸². In this context, the non-classical molecular chaperone Hsp47, also known as serpin H1, which is essential for biosynthesis and secretion of collagen molecules has been identified in the sera of rheumatoid arthritis (RA) patients ^{183, 184}. Elevated serum levels of Hsp47 were also present in patients with mixed connective tissue disease ¹⁸⁵, acute exacerbation (AE) of idiopathic pulmonary fibrosis ¹⁸⁶ or acute idiopathic interstitial pneumonias ¹⁸⁷. Work by the group of Shigeru Kohno convincingly revealed significantly elevated levels of anti-Hsp40 autoantibodies in the periphery of patients with lung cancer ¹⁸⁸ and those with ulcerative colitis campared to normal individuals ¹⁸⁹. Elevated levels of anti-DnaJ, anti-DnaJA1/Hdj2, and anti-DnaJC14/Hdj3 serum antibodies could also be detected in RA patients ^{15, 190}.

Following environmental stress, expression of DNAJs/HSP40s is dramatically upregulated in order to promote cell survival in the face of endogenous or exogenous injury. In this regard, exposure of cells to the chemicals ibuprofen, indomethacin, ZnSO₄, and 8-hydroxy-quinoline induced expression of Hsp40/DnaJB1 with concomitant translocation to the nucleus ¹⁹¹. Moreover, the ER stress gene DNAJC10/ERDJ5 was found to be induced in neuroectodermal tumor cells by the retinoid analog fenretinide ¹⁹², which has an increasingly important profile as a cancer preventive and chemotherapeutic drug ¹⁹³. Chemical stress also induces release of DNAJs/HSP40s into the extracellular milieu. As demonstrated by Saito and colleagues, treatment of cells with acrylamide caused necrotic and thus irreversible cell death accompanied by the release of several HSPs including DNAJs/HSP40s ¹⁹⁴.