Sally‐Lin Adams

Mapping protein-protein interactions is an invaluable tool for understanding protein function. Here, we report the first large-scale study of protein-protein interactions in human cells using a mass spectrometry-based approach. The study maps protein interactions for 338 bait proteins that were selected based on known or suspected disease and functional associations. Large-scale immunoprecipitation of Flag-tagged versions of these proteins followed by LC-ESI-MS/MS analysis resulted in the identification of 24,540 potential protein interactions. False positives and redundant hits were filtered out using empirical criteria and a calculated interaction confidence score, producing a data set of 6463 interactions between 2235 distinct proteins. This data set was further cross-validated using previously published and predicted human protein interactions. In-depth mining of the data set shows that it represents a valuable source of novel protein-protein interactions with relevance to human diseases. In addition, via our preliminary analysis, we report many novel protein interactions and pathway associations.

Elongation factor P (EFP) is a protein that stimulates the peptidyltransferase activity of fully assembled 70 S prokaryotic ribosomes and enhances the synthesis of certain dipeptides initiated by N-formylmethionine. This reaction appears conserved throughout species and is promoted in eukaryotic cells by a homologous protein, eIF5A. Here we ask whether the Escherichia coli gene encoding EFP is essential for cell viability. A kanamycin resistance (KanR) gene was inserted near the N-terminal end of the efp gene and was cloned into a plasmid, pMAK705, that has a temperature-sensitive origin of replication. After transformation into a recA+ E. coli strain, temperature-sensitive mutants were isolated, and their chromosomal DNA was sequenced. Mutants containing the efp-KanR gene in the chromosome grew at 33 degrees C only in the presence of the wild-type copy of the efp gene in the pMAK705 plasmid and were unable to grow at 44 degrees C. Incorporation of various isotopes in vivo suggests that translation is impaired in the efp mutant at 44 degrees C. At 44 degrees C, mutant cells are severely defective in peptide-bond formation. We conclude that the efp gene is essential for cell viability and is required for protein synthesis.

Shwachman-Diamond Syndrome (SDS) is a multi-system genetic disorder with bone marrow failure. SBDS, the gene associated with SDS, has been postulated to play a role in ribosome biogenesis and RNA processing, but its functions are still unknown. To study whether these pathways are interrupted when Sbds protein is lost, we studied the expression of related genes in patient SBDS-/- cells by an oligonucleotide microarray. We first analysed ribosomal protein (RP) genes, which are normally co-regulated. In SDS, 27 of the 85 RP genes were downregulated. Among the downregulated RP genes, seven are known to be associated with the inhibition of apoptosis. RPS27L, which mediates p53-dependent induction of apoptosis, was the only upregulated RP gene. Interestingly, several genes involved in RP mRNA transcription were downregulated without affecting the expression of genes involved in mRNA degradation, suggesting that the downregulation of the RP gene expression might be at the transcriptional level. Importantly we also found dysregulation of multiple genes involved in rRNA transcription and pre-rRNA processing. We conclude that SDS marrow cells exhibit major dysregulation of RP, RNA processing and RNA transcription genes.

A soluble protein EF-P (elongation factor P) from Escherichia coli has been purified and shown to stimulate efficient translation and peptide-bond synthesis on native or reconstituted 70S ribosomes in vitro. Based on the partial amino acid sequence of EFP, 18- and 24-nucleotide DNA probes were synthesized and used to screen λ phage clones from the Kohara Gene Bank. The entire EF-P gene was detected on λ clone #650 which contains sequences from the 94 minute region of the E.coli genome. Two DNA fragments, 3.0 and 0.78 kilobases in length encompassing the gene, were isolated and cloned into pUC18 and pUC19. Partially purified extracts from cells transformed with these plasmids overrepresented a protein which co-migrates with EF-P upon SDS polyacrylamide gel electrophoresis, and also exhibited increased EF-P mediated peptide-bond synthetic activity. Based on DNA sequence analysis of this gene, the EF-P protein consists of 187 amino acids with a calculated molecular weight of 20,447. The sequence and chromosomal location of EF-P establishes it as a unique gene product.