Voltage-dependent anion-selective channel (VDAC) is a b-barrel protein in the outer mitochondrial membrane that is necessary for metabolite exchange with the cytosol and is proposed to be involved in certain forms of apoptosis. We studied the biogenesis of VDAC in human mitochondria by depleting the components of the mitochondrial import machinery by using RNA interference. Here, we show the importance of the translocase of the outer mitochondrial membrane (TOM) complex in the import of the VDAC precursor. The deletion of Sam50, the central component of the sorting and assembly machinery (SAM), led to both a strong defect in the assembly of VDAC and a reduction in the steadystate level of VDAC. Metaxin 2-depleted mitochondria had reduced levels of metaxin 1 and were deficient in import and assembly of VDAC and Tom40, but not of three matrix-targeted precursors. We also observed a reduction in the levels of metaxin 1 and metaxin 2 in Sam50-depleted mitochondria, implying a connection between these three proteins, although Sam50 and metaxins seemed to be in different complexes. We conclude that the pathway of VDAC biogenesis in human mitochondria involves the TOM complex, Sam50 and metaxins, and that it is evolutionarily conserved.

A 'writer-reader-eraser' post-translational modification regulatory system consisting of a large number of methyltransferases 8,9 , methyl-recognition domain-containing proteins 10 and putative demethylases 11 are expressed in different subcellular locations in humans, an indication that protein methylation is involved in processes other than epigenetic regulation.We prepared 82 Y2H bait strains spanning human R-methyltransferases (PRMT1-PRMT8) 8 , 16 SET domaincontaining K-methyltransferases (PKMTs) 9 , 9 members of the JMJD domain-containing protein family of protein demethylases and AOF2 (LSD1) 11 (Supplementary Table 1). In our current matrix screening protocol 4,12 , we perform four replicates, testing every set of baits individually against each of the ~13,000 prey contained in the matrix. Interacting prey are identified by their position in the matrix. To increase the sensitivity of the approach while also reducing the workload, we used a pooled strategy to test each protein pair substantially more than four times. Baits were pooled with all prey strains and then assayed for interaction in more than 100,000 separate spots ( Fig. 1a and Supplementary Fig. 1). Using Y2H-seq, we obtained 4-10 times the number of positive colonies obtained with the matrix approach. To reveal the prey identities, we collected all colonies and performed a 36-base parallel sequencing run. More than 20 million reads mapped perfectly to human RefSeq coding sequences (open reading frames, ORFs), corresponding to more than 500,000 unique 36-base reads (Supplementary Table 2). To rank the potentially interacting proteins for subsequent interaction retesting, we calculated a 'SeqScore' that incorporates the number of total mappings and the number of unique reads matching the ORF ( Supplementary Fig. 2). Notably, >99.7% of the RefSeq mappings matched to the 400 top-ranked genes, thus allowing the identification of potentially interacting ORFs with an extremely high signal-tobackground ratio (Supplementary Table 2).We performed four biological replicates and demonstrated in statistical pairwise comparisons that they result in very similar ranked prey orders (Supplementary Fig. 3). Top-ranked prey in at least two replicate screens were retested against all baits in a pairwise manner (Supplementary Fig. 4) and yielded 463 protein interactions (Supplementary Table 3). The success rate of the retest-that is, the probability that the prey is interactingdecreased with decreasing SeqScore (Fig. 1b).We also performed a matrix screen in quadruplicate with a subset of the protein methyltransferase (PMT) and protein demethylase (PDeM) baits for direct method comparison. With the matrix approach, we found 151 interactions (Supplementary to accelerate high-density interactome mapping, we developed a yeast two-hybrid interaction screening approach involving short-read second-generation sequencing (Y2h-seq) with improved sensitivity and a quantitative scoring readout allowing rapid interaction validation. We applied Y2h-seq to investigate enzymes in...

Post-translational protein modifications, such as tyrosine phosphorylation, regulate protein–protein interactions (PPIs) critical for signal processing and cellular phenotypes. We extended an established yeast two-hybrid system employing human protein kinases for the analyses of phospho-tyrosine (pY)-dependent PPIs in a direct experimental, large-scale approach. We identified 292 mostly novel pY-dependent PPIs which showed high specificity with respect to kinases and interacting proteins and validated a large fraction in co-immunoprecipitation experiments from mammalian cells. About one-sixth of the interactions are mediated by known linear sequence binding motifs while the majority of pY-PPIs are mediated by other linear epitopes or governed by alternative recognition modes. Network analysis revealed that pY-mediated recognition events are tied to a highly connected protein module dedicated to signaling and cell growth pathways related to cancer. Using binding assays, protein complementation and phenotypic readouts to characterize the pY-dependent interactions of TSPAN2 (tetraspanin 2) and GRB2 or PIK3R3 (p55γ), we exemplarily provide evidence that the two pY-dependent PPIs dictate cellular cancer phenotypes.