Colloidal silver staining of electroblotted proteins for high sensitivity peptide mapping by liquid chromatography-electrospray ionization tandem mass spectrometry

Mass spectrometric techniques for the identification of proteins either by amino acid sequencing or by correlation of mass spectral data with sequence databases are becoming increasingly sensitive and are rapidly approaching the limit of detection achieved by the staining of proteins in gels or, after electroblotting, on membranes. Here we present a technique for the sensitive staining of proteins electroblotted onto nitrocellulose or polyvinylidene difluoride membranes and enzymatic cleavage conditions for such proteins to achieve optimal recovery of peptides. The technique is based on the deposition of colloidal silver on the membrane-bound proteins. Peptide mixtures generated by proteolysis on the membrane were recovered at high yields and were compatible with analysis by reverse-phase chromatography and on-line electrospray ionization mass spectrometry. This simple and rapid colloidal silver staining procedure allowed the visualization of less than 5 ng of protein in a band and thus approached the sensitivity of silver staining in gels. We demonstrate that this method allows the detection of subpicomole amounts of electroblotted proteins and their identification by high-performance liquid chromatography-electrospray ionization tandem mass spectrometry.

Protein identification by capillary zone electrophoresis/microelectrospray ionization-tandem mass spectrometry at the subfemtomole level

A method for the identification of proteins by their amino acid sequence at the low-femtomole to subfemtomole sensitivity level is described. It is based on an integrated system consisting of a capillary zone electrophoresis (CZE) instrument coupled to an electrospray ionization triple- quadrupole tandem mass spectrometer (ESI-MS/MS) via a microspray interface. The method consists of proteolytic fragmentation of a protein, peptide separation by CZE, analysis of separated peptides by ESI-MS/MS, and identification of the protein by correlation of the collision-induced dissociation (CID) patterns of selected peptides with the CID patterns predicted from all the isobaric peptides in a sequence database. Using standard peptides applied to a 20-microns-i.d. capillary, we demonstrate an ESI-MS limit of detection of less than 300 amol and CID spectra suitable for searching sequence databases obtained with 600 amol of sample applied to the capillary. Successful protein identification by the method was demonstrated by applying 50 and 38 fmol of a tryptic digest of the proteins beta-lactoglobulin and bovine serum albumin, respectively, to the system.

Activation and serine phosphorylation of the p56lck protein tyrosine kinase in response to antigen receptor cross-linking in B lymphocytes

We show that cross-linking the B cell AgR with anti-Ig Abs activates p56lck (Lck) in both the immature B cell line WEHI-231 and mature resting B cells from mouse spleen. Anti-Ig-stimulated Lck activity peaked after 1 to 2 min, but remained elevated for at least 15 min. Consistent with the proposed role for src family tyrosine kinases in AgR signaling, we found that Lck could phosphorylate the cytoplasmic tails of the Ig-alpha and Ig-beta components of the B cell AgR in vitro. Lck phosphorylated both of the tyrosines in the Ig-beta AgR homology motif and one of the two tyrosines in the Ig-alpha AgR homology motif. Finally, we show that AgR ligation in B cells caused a significant portion of the Lck to migrate with an apparent molecular mass of 60 kDa on SDS-PAGE gels. Conversion of p56lck to p60lck was maximal at 5 to 15 min, at which times Lck activity in the cells was decreasing. This Lck "band shift" has been observed previously in activated T cells and correlates with phosphorylation of Lck at serine 59. We show that the 60-kDa form of Lck induced by AgR cross-linking in B cells is also phosphorylated at serine 59. Phosphorylation of Lck at this site in vitro decreases its activity. Thus, in B cells, AgR cross-linking activates Lck and subsequently activates a kinase that phosphorylates Lck at serine 59, a potential negative regulatory site.

Activation and serine phosphorylation of the p56lck protein tyrosine kinase in response to antigen receptor cross-linking in B lymphocytes

We show that cross-linking the B cell AgR with anti-Ig Abs activates p56lck (Lck) in both the immature B cell line WEHI-231 and mature resting B cells from mouse spleen. Anti-Ig-stimulated Lck activity peaked after 1 to 2 min, but remained elevated for at least 15 min. Consistent with the proposed role for src family tyrosine kinases in AgR signaling, we found that Lck could phosphorylate the cytoplasmic tails of the Ig-alpha and Ig-beta components of the B cell AgR in vitro. Lck phosphorylated both of the tyrosines in the Ig-beta AgR homology motif and one of the two tyrosines in the Ig-alpha AgR homology motif. Finally, we show that AgR ligation in B cells caused a significant portion of the Lck to migrate with an apparent molecular mass of 60 kDa on SDS-PAGE gels. Conversion of p56lck to p60lck was maximal at 5 to 15 min, at which times Lck activity in the cells was decreasing. This Lck "band shift" has been observed previously in activated T cells and correlates with phosphorylation of Lck at serine 59. We show that the 60-kDa form of Lck induced by AgR cross-linking in B cells is also phosphorylated at serine 59. Phosphorylation of Lck at this site in vitro decreases its activity. Thus, in B cells, AgR cross-linking activates Lck and subsequently activates a kinase that phosphorylates Lck at serine 59, a potential negative regulatory site.

B cell antigen receptor cross-linking induces phosphorylation of the p21ras oncoprotein activators SHC and mSOS1 as well as assembly of complexes containing SHC, GRB-2, mSOS1, and a 145-kDa tyrosine-phosphorylated protein

Ligation of the B cell AgR activates p21ras (Ras). We have investigated the effects of AgR ligation on three proteins that have been implicated as regulators of Ras: SHC, GRB-2, and mSOS1. We show that AgR cross-linking in B cells stimulated tyrosine and serine phosphorylation of SHC. This correlated with the formation of complexes containing SHC, GRB-2, mSOS1, and an unidentified 145-kDa tyrosine-phosphorylated protein. These complexes were present in the cytosol, as well as in the membrane fraction of the cells, where Ras is located. By using a GRB-2 fusion protein to probe blots, we showed that SHC was the major protein that GRB-2 bound to in anti-Ig-stimulated B cells. This argues that SHC couples GRB-2/mSOS1 to the 145-kDa protein and that SHC is likely to be essential for mSOS1 function in B cells. Finally, we found that AgR cross-linking stimulated phosphorylation of mSOS1 and that this could be blocked by an inhibitor of protein kinase C. Thus, signaling by the B cell AgR stimulates phosphorylation of SHC and mSOS1 and induces the formation of membrane-associated complexes containing SHC, GRB-2, mSOS1, and a 145-kDa protein. These events may be important for activation of Ras by the AgR.

B cell antigen receptor cross-linking induces phosphorylation of the p21ras oncoprotein activators SHC and mSOS1 as well as assembly of complexes containing SHC, GRB-2, mSOS1, and a 145-kDa tyrosine-phosphorylated protein

Ligation of the B cell AgR activates p21ras (Ras). We have investigated the effects of AgR ligation on three proteins that have been implicated as regulators of Ras: SHC, GRB-2, and mSOS1. We show that AgR cross-linking in B cells stimulated tyrosine and serine phosphorylation of SHC. This correlated with the formation of complexes containing SHC, GRB-2, mSOS1, and an unidentified 145-kDa tyrosine-phosphorylated protein. These complexes were present in the cytosol, as well as in the membrane fraction of the cells, where Ras is located. By using a GRB-2 fusion protein to probe blots, we showed that SHC was the major protein that GRB-2 bound to in anti-Ig-stimulated B cells. This argues that SHC couples GRB-2/mSOS1 to the 145-kDa protein and that SHC is likely to be essential for mSOS1 function in B cells. Finally, we found that AgR cross-linking stimulated phosphorylation of mSOS1 and that this could be blocked by an inhibitor of protein kinase C. Thus, signaling by the B cell AgR stimulates phosphorylation of SHC and mSOS1 and induces the formation of membrane-associated complexes containing SHC, GRB-2, mSOS1, and a 145-kDa protein. These events may be important for activation of Ras by the AgR.

Purification and identification of tyrosine-phosphorylated proteins from B lymphocytes stimulated through the antigen receptor

The activation of protein tyrosine kinase (PTKs) and subsequent tyrosine phosphorylation of cellular proteins is a critical initial signal in the response of eukaryotic cells to mitogens, differentiative signals, and other stimuli. A number of PTK substrates have been identified and many of these are components of signal transduction pathways that regulate cell function. However, the majority of proteins that are tyrosine-phosphorylated in response to receptor signaling remain unidentified. As some of these unidentified PTK substrates may also be signal-transducing proteins, their identification and functional characterization is an important objective towards understanding receptor signaling. We describe the development of a comprehensive and general process for the isolation and structural characterization of tyrosine-phosphorylated proteins. The method involves enrichment by anti-phosphotyrosine affinity chromatography, electrophoretic concentration and separation, and proteolytic fragmentation of individual purified phosphoproteins. Resulting peptide fragments are separated by microbore reverse-phase high performance liquid chromatography (RP-HPLC) and a portion of the eluted peptides are subjected to electrospray-mass spectrometry (ES/MS) for accurate determination of peptide masses. Proteolytic fragmentation of a protein produces a characteristic set of peptide masses that can be used to rapidly identify the protein by searching databases containing the peptide mass "fingerprints" for all known proteins. The identity of the protein established by this method can be confirmed by sequence analysis of selected peptides. We have applied this procedure to the analysis of PTK substrates from B lymphocytes that have been stimulated through the B cell antigen receptor (BCR). Signaling by this receptor is involved in the generation of antibodies against foreign molecules (antigens). The BCR activates multiple PTKs which phosphorylate at least 30 different proteins. We have identified several of these tyrosine-phosphorylated proteins, including Syk, a PTK that is known to be tyrosine-phosphorylated in activated B cells. Thus, the procedure described here can be used to identify regulatory proteins of low abundance. The process consists of a logical succession of compatible steps that avoids pitfalls inherent to prior attempts to characterize low abundance phosphoproteins and should find wide use for the identification of tyrosine-phosphorylated proteins in other cell types.