TMEM85 (Transmembrane Protein 85)
Prior to 2008, there were no publications on the TMEM85 gene or protein. The GenBank and Entrez Gene databases described TMEM85 as an evolutionary conserved transmembrane domain containing protein (DUF1077 superfamily motif) found in all eukaryotes examined ( TMEM85 (Transmembrane Protein 85)). In the yeast Saccharomyces cerevisiae, the TMEM85 orthologue is called YGL231c. In 2003, it was reported that yeast cells lacking YGL231c showed an increased sensitivity to the toxic effects of ectopically expressed Parkinson’s disease associated gene α-synuclein (Willingham et al. 2003). Finally, the analysis of a GFP tagged genomic copy of YGL231c suggests that the protein is a resident of the endoplasmic reticulum (ER) (YGL231c-gene).
TMEM85, An Anti-Apoptotic Gene
By screening a human heart cDNA expression library in yeast cell undergoing apoptosis due to the ectopic expression of a cDNA encoding the pro-apoptotic Bax protein, Ring et al. (2008) identified human TMEM85 as a Bax suppressor. Bax is the pro-apoptotic member of the Bcl-2 family of proteins. Such yeast-based screens have been useful in identifying a variety of different anti-apoptotic sequences (Greenwood and Ludovico 2010). For example, the ceramide utilizing sphingomyelin synthase was originally identified in such a screen.
Ectopic expression of TMEM85 also protected yeast from the apoptotic inducing effects of Reactive Oxygen Species (ROS). This suggests that TMEM85 is not simply a Bax suppressor but an anti-apoptotic gene. Overexpression of the yeast TMEM85 orthologue also protected yeast cells from apoptotic inducing stress (Ring et al. 2008).
TMEM85 Structure and Function
Human TMEM85 is a complex alternatively spliced gene that is reported to produce four different transcripts and proteins (Ring et al. 2008). The predicted proteins range in size from 183, 149, 97, and 66 residues. The largest protein is predicted to contain two transmembrane domains and it likely corresponds to the yeast 190 residue YGL231c ortholog. The 149 residue TMEM85 variant (v2) is identical to the 183 residue protein at its N-terminal but it contains a unique C-terminal region and a single transmembrane domain. It represents the sequence identified as having anti-apoptotic properties. The other variant proteins are predicted to be soluble proteins.
There are other discernable functional sequences present within the TMEM85 proteins. Thus the structure provides no insight to what its function could be in the ER or how it may function as an anti-apoptotic protein (UniProt page). Although many anti-apoptotic proteins, such as heat shock proteins and Bcl-2, belong to large family of proteins many others are unique or small groups of proteins, such as heme oxygenase 1, whose anti-apoptotic effects cannot be readily deciphered from their sequence. This lack of functional information is somewhat surprising considering that TMEM85 is highly conserved and is present in species from yeast to man ranging all way from worms to plants. There are a large number of web sites that have compiled the TMEM85 sequences from different species (http://omabrowser.org/cgi-bin/gateway.pl?f = DisplayEntry&p1 = 0061323).
One interesting study, using a two hybrid screen, identified TMEM85 as protein that interacts with protein prestin (Zheng et al. 2009). Prestin is a component of the outer hair cells (OHC) that are housed in the organ of Corti and play a mechanosensing role important for hearing. Knockout mouse models of prestin clearly show that they play an important role in the process of hearing. The fact that TMEM85 is highly conserved and is present in unicellular organisms like yeast suggest that it must play a fundamental role in the cell, while its potential interaction with more specialized proteins like preston suggests that TMEM85 may also have multiple functions in protozoans.
Very little is currently published regarding the expression or regulation of the mammalian TMEM85 gene. RT-PCR analysis of the RNA isolated from a variety of tissues suggests that all four transcripts are expressed in the heart while variant 1 and 4 are commonly observed in most tissues (Ring et al. 2008). A few other reports that have studied global gene expression patterns have identified TMEM85 expression to be increased in the Caudate Nucleus of Parkinson’s patients as well as being more highly expressed in the brain white matter compared to the spinal cord (Bossers et al. 2009; Yan et al. 2009).
Function of the TMEM85 Orthologue in Yeast
Nothing else is known about mammalian TMEM85, so further clues as to the potential function of TMEM85 may be obtained from a recent study in yeast that carried out a global analysis to identify proteins that are required for protein folding in the ER (Jonikas et al. 2009). Of interest, the yeast orthologue of TMEM85, YGL231c, was identified as being part of tight multiprotein transmembrane complex of six poorly characterized proteins. Although the function of the complex was not determined, all six members were renamed as EMC1 to 6 proteins (YGL231c is EMC4p) to reflect that they are part of an ER membrane protein complex that is likely to have a functional significance. Cells lacking the complex show increased accumulation of misfolded ER proteins. Given that misfolded proteins leads to ER stress and to the induction of apoptosis, the proposed function of the EMC complex is in line with the reported anti-apoptotic function of TMEM85.
Human TMEM85 represents a complex alternatively spliced gene that encodes highly conserved transmembrane domain proteins of largely unknown function. TMEM85 was identified as a protein with anti-apoptotic properties in a screen of yeast cells ectopically expressing a pro-apoptotic mammalian Bax sequence. Analysis of the yeast ortholog of TMEM85, YGL231c, suggests that it is a protein that resides in the endoplasmic reticulum (ER). Further analysis suggests that YGL231c forms a tight complex with five other also largely functionally uncharacterized proteins. YGL231c was renamed ECM4 to reflect the fact that it is found in this ER membrane protein complex. The proposed role of the ECM complex in regulating folding of ER proteins is consistent with the reported anti-apoptotic function of human TMEM85. A significant amount of work is still required in order to understand the role of TMEM85.
- Saccharomyces Genome Database for the YGL231c gene. http://www.yeastgenomeorg/cgi-bin/locusfpl?locus=YGL231C. Accessed 5 April 2011.Google Scholar
- The Entez Gene web page for the TMEM85 gene. http://www.ncbinlmnihgov/gene/51234. Accessed 5 April 2011.
- The Universal Protein Resource (UniProt) web page for the human TMM85 protein http://www.uniprot.org/uniprot/Q5J8M3. Accessed 5 April 2011.