Samp1, a RanGTP binding transmembrane protein in the inner nuclear membrane

A proteomic approach of biomarker candidate discovery for alcoholic liver cirrhosis

Alcoholic liver disease (ALD) progresses from steatosis to alcoholic hepatitis to fibrosis and cirrhosis. Liver biopsy is considered as the gold standard method for diagnosis of liver cirrhosis and provides useful information about damaging process which is an invasive procedure with complications. Existing biomarkers in clinical practice have narrow applicability due to lack of specificity and lack of sensitivity. The objective of this article is to identify proteomic biomarker candidates for alcoholic liver cirrhosis by differential expression analysis between alcoholic liver cirrhotic and healthy subjects. Blood samples were collected from 20 subjects (10 alcoholic liver cirrhosis and 10 healthy) from R. L. Jalapa Hospital and Research Centre, Kolar, Karnataka, India. Differential protein analysis was carried out by two-dimensional electrophoresis after albumin depletion, followed by liquid chromatography–mass spectrometry. The image analysis found 46 spots in cirrhotic gel and 69 spots in healthy gel, of which 14 spots were identified with significant altered expression levels. Based on the protein score and clinical significance, among 14 spots, a total of 28 protein biomarker candidates were identified: 13 with increased expression and 15 with decreased expression were categorized in alcoholic liver cirrhosis compared to healthy subjects. Protein biomarker candidates identified by “-omics” approach based on differential expression between alcoholic liver cirrhotic subjects and healthy subjects may give better insights for diagnosis of ALD. Prioritization of candidates identified is a prerequisite for validation regimen. Biomarker candidates require verification that demonstrates the differential expression will remain detectable by assay to be used for validation.

Mitotic spindle assembly and γ-tubulin localisation depend on the integral nuclear membrane protein Samp1

We have investigated a possible role for the inner nuclear membrane protein Samp1 (also known as TMEM201) in the mitotic machinery. Live-cell imaging showed that Samp1a–YFP (Samp1a is the short isoform of Samp1) distributed as filamentous structures in the mitotic spindle, partially colocalising with β-tubulin. Samp1 depletion resulted in an increased frequency of cells with signs of chromosomal mis-segregation and prolonged metaphase, indicating problems with spindle assembly and/or chromosomal alignment. Consistent with this, mitotic spindles in Samp1-depleted cells contained significantly lower levels of β-tubulin and γ-tubulin, phenotypes that were rescued by overexpression of Samp1a–YFP. We found that Samp1 can bind directly to γ-tubulin and that Samp1 co-precipitated with γ-tubulin and the HAUS6 subunit of the Augmin complex in live cells. The levels of HAUS6, in the mitotic spindle also decreased after Samp1 depletion. We show that Samp1 is involved in the recruitment of HAUS6 and γ-tubulin to the mitotic spindle. Samp1 is the first inner nuclear membrane protein shown to have a function in mitotic spindle assembly.

Highlighted Article: The transmembrane inner nuclear membrane protein Samp1 has a functional role in recruitment of γ-tubulin to the mitotic spindle and correct spindle assembly.

RanGTPase regulates the interaction between the inner nuclear membrane proteins, Samp1 and Emerin

Samp1, spindle associated membrane protein 1, is a type II integral membrane protein localized in the inner nuclear membrane. Recent studies have shown that the inner nuclear membrane protein, Emerin and the small monomeric GTPase, Ran are direct binding partners of Samp1. Here we addressed the question whether Ran could regulate the interaction between Samp1 and Emerin in the inner nuclear membrane. To investigate the interaction between Samp1 and Emerin in live cells, we performed FRAP experiments in cells overexpressing YFP-Emerin. We compared the mobility of YFP-Emerin in Samp1 knock out cells and cells overexpressing Samp1. The results showed that the mobility of YFP-Emerin was higher in Samp1 knock out cells and lower in cells overexpressing Samp1, suggesting that Samp1 significantly attenuates the mobility of Emerin in the nuclear envelope. FRAP experiments using tsBN2 cells showed that the mobility of Emerin depends on RanGTP. Consistently, in vitro binding experiments showed that the affinity between Samp1 and Emerin is decreased in the presence of Ran, suggesting that Ran attenuates the interaction between Samp1 and Emerin. This is the first demonstration that Ran can regulate the interaction between two proteins in the nuclear envelope.