Jason Ptacek

Global analysis of protein phosphorylation in yeast

Protein phosphorylation is estimated to affect 30% of the proteome and is a major regulatory mechanism that controls many basic cellular processes. Until recently, our biochemical understanding of protein phosphorylation on a global scale has been extremely limited; only one half of the yeast kinases have known in vivo substrates and the phosphorylating kinase is known for less than 160 phosphoproteins. Here we describe, with the use of proteome chip technology, the in vitro substrates recognized by most yeast protein kinases: we identified over 4,000 phosphorylation events involving 1,325 different proteins. These substrates represent a broad spectrum of different biochemical functions and cellular roles. Distinct sets of substrates were recognized by each protein kinase, including closely related kinases of the protein kinase A family and four cyclin-dependent kinases that vary only in their cyclin subunits. Although many substrates reside in the same cellular compartment or belong to the same functional category as their phosphorylating kinase, many others do not, indicating possible new roles for several kinases. Furthermore, integration of the phosphorylation results with protein–protein interaction and transcription factor binding data revealed novel regulatory modules. Our phosphorylation results have been assembled into a first-generation phosphorylation map for yeast. Because many yeast proteins and pathways are conserved, these results will provide insights into the mechanisms and roles of protein phosphorylation in many eukaryotes.

Protein Microarray Technology

Abstract Protein chips have emerged as a promising approach for a wide variety of applications including the identification of protein–protein interactions, protein–phospholipid interactions, small molecule targets, and substrates of proteins kinases. They can also be used for clinical diagnostics and monitoring disease states. This article reviews current methods in the generation and applications of protein microarrays.

Substrate specificity analysis of protein kinase complex Dbf2-Mob1 by peptide library and proteome array screening

BackgroundThe mitotic exit network (MEN) is a group of proteins that form a signaling cascade that is essential for cells to exit mitosis in Saccharomyces cerevisiae. The MEN has also been implicated in playing a role in cytokinesis. Two components of this signaling pathway are the protein kinase Dbf2 and its binding partner essential for its kinase activity, Mob1. The components of MEN that act upstream of Dbf2-Mob1 have been characterized, but physiological substrates for Dbf2-Mob1 have yet to be identified.ResultsUsing a combination of peptide library selection, phosphorylation of opitmal peptide variants, and screening of a phosphosite array, we found that Dbf2-Mob1 preferentially phosphorylated serine over threonine and required an arginine three residues upstream of the phosphorylated serine in its substrate. This requirement for arginine in peptide substrates could not be substituted with the similarly charged lysine. This specificity determined for peptide substrates was also evident in many of the proteins phosphorylated by Dbf2-Mob1 in a proteome chip analysis.ConclusionWe have determined by peptide library selection and phosphosite array screening that the protein kinase Dbf2-Mob1 preferentially phosphorylated substrates that contain an RXXS motif. A subsequent proteome microarray screen revealed proteins that can be phosphorylated by Dbf2-Mob1 in vitro. These proteins are enriched for RXXS motifs, and may include substrates that mediate the function of Dbf2-Mob1 in mitotic exit and cytokinesis. The relatively low degree of sequence restriction at the site of phosphorylation suggests that Dbf2 achieves specificity by docking its substrates at a site that is distinct from the phosphorylation site

Global analysis of protein function using protein microarrays

Protein microarrays containing thousands of proteins arrayed at high density can be prepared and probed for a wide variety of activities, thereby allowing the large scale analysis of many proteins simultaneously. In addition to identifying the activities of many previously uncharacterized proteins, protein microarrays can reveal new activities of well-characterized proteins, thus providing new insights about the functions of these proteins. Below, we describe the construction and use of protein microarrays and their applications using yeast as a model system.

Kinase Substrate Interactions

Kinases have become popular therapeutic targets primarily due to their integral role in cell cycle and tumor progression. The efficacy of high-throughput screening efforts is dependent on the development of high quality multiplex tools capable of replacing lower-throughput technologies such as mass spectroscopy or solution-based assays for the study of kinase-substrate interactions. Functional protein microarrays are comprised of thousands of immobilized proteins on glass slides that have been used successfully to identify protein-protein interactions. Here, we describe the application of functional protein microarrays for the identification of the phosphorylation targets of individual protein kinases using highly sensitive radioactive detection and robust informatics algorithms.

14 Yeast Protein Microarrays

Publisher Summary Protein microarrays are the arrays of protein, or in the case of yeast, nearly the entire proteome, which will expedite the study of the proteome by providing a platform to elucidate a proteins function and the way it relates to other proteins on a global scale. This chapter discusses many challenges and options that exist in designing a yeast protein array and the many questions that have been addressed using this technology, predominantly in the form of functional protein microarrays. All technologies involving proteins are challenged by the large scale of the proteome and the difficulty in working with proteins, given that their chemistry and solubility are much more variable. The goals of proteomics and a sample of the common technologies applied to each are listed in tabulated form in the chapter. Many techniques have been used to address different aspects of these goals. Mass spectrometry has been used to identify protein complexes, the components of the yeast nuclear pore complex, and to catalogue 1484 proteins from yeast in log-phase. By using this technique, it is difficult, but not impossible, to determine if a protein interaction is a direct, or binary, interaction or if it is an indirect interaction, mediated by other components of the complex.

Workshop I – Global Analysis of Protein Activities Using Protein Chips

The genomes of a wide variety of organisms have now been sequenced; a major challenge ahead is to understand the function, regulation and modification of the many encoded gene products. We have been carrying out proteomics approaches to the identification and analysis of signalling pathways in yeast. 121 of 122 protein kinases were cloned and purifed from yeast as GST fusions and analyzed for their ability to phosphorylate 60 different yeast substrates. More than 93% of the kinases exhibited activities that are 5 fold or higher, relative to controls, including 18 of 24 previously uncharacterized kinases. Many protein kinases had novel activities; for example 27 yeast kinases were found to phosphorylate Tyr. In addition, we have now cloned 6000 open reading frames and overexpressed their corresponding proteins. The proteins were printed onto slides at high spatial density to form a yeast proteome microarray and screened for their ability to interact with a variety of different proteins, nucleic acids and phospholipids. As examples, we have probed yeast proteome chips with calmodulin and six different phospholipids. These studies revealed many new calmodulin and phospholipid-interacting proteins; a common potential binding motif was identified for many of the calmodulin-binding proteins. Thus, microarrays of an entire eukaryotic proteome can be prepared and screened for diverse biochemical activities. They can also be used to screen protein-drug interactions and to detect posttranslational modifications.