Requirements for phosphorylation of MAP kinase during meiosis in Xenopus oocytes

Mitogen-activated protein (MAP) kinases are activated in response to a variety of extracellular stimuli by phosphorylation on tyrosine and threonine residues. Xp42 is a Xenopus laevis MAP kinase that is activated during oocyte maturation. Modified forms of Xp42 that lacked enzymatic activity or either of the phosphorylation sites were expressed in Xenopus oocytes. When meiotic maturation was induced with progesterone, each mutant Xp42 was phosphorylated, indicating that at least one kinase was activated that can phosphorylate Xp42 on tyrosine and threonine. Phosphorylation of one residue is not strictly dependent on phosphorylation of the other.

Diagnosis of sternal wound infection by technetium-99m-leukocyte imaging

An imaging study is needed that can detect sternal wound infections and distinguish between superficial and deep sternal wound infection when a clinical diagnosis is uncertain and a decision regarding surgical intervention must be made. We retrospectively reviewed the 99mTc-leukocyte scans of 29 patients referred to rule out sternal wound infection. The presence or absence of deep or superficial sternal wound infection was determined by microbiology and long-term follow-up. Images obtained 4 and 20 hr after injection were reviewed by two nuclear physicians who were blinded to the clinical history. Findings were categorized as normal or abnormal. Abnormal images were further defined as having intense uptake at 4 and 20 hr, increasing uptake between 4 and 20 hr, or other patterns such as focal cold regions, irregular uptake at 4 and 20 hr or increasing uptake between 4 and 20 hr were 100% sensitive and 89% specific for the detection of deep sternal wound infection. The images were also useful for determining the extent of infection. Superficial sternal wound infection could not be reliably detected. The results indicate that 99mTc-leukocyte imaging is useful for the diagnosis of deep sternal wound infection.

Cognitive impairment in early, untreated Parkinson's disease and its relationship to motor disability

Current knowledge of cognitive dysfunction in Parkinson's disease (PD) has largely been obtained from studies of chronically treated patients in whom effects of disease chronicity, treatment, depression and dementia are confounding factors. Studies of untreated patients have examined few cognitive domains and relationships between cognition, depression and motor disability have been incompletely explored. Accordingly, we studied 60 consecutive patients with newly diagnosed, untreated, idiopathic PD and 37 matched, healthy control subjects; no subject had clinical dementia or depression. All subjects received tests of specific processes of memory and cognition, including working memory, verbal and non-verbal short- and long-term memory, language, visuospatial capacity, set-formation and shifting and sequencing. Patients also received quantitative global clinical measures of severity of dementia, depression and motor disability. The PD group as a whole showed deficits in immediate recall of verbal material, language production and semantic fluency, set-formation, cognitive sequencing and working memory and visuomotor construction. However, this group was unimpaired in immediate memory span, long-term forgetting, naming, comprehension and visual perception. Language deficits and more severe frontal lobe impairments were confined to those PD patients scoring abnormally on a Mini Mental State examination. Motor disability correlated strongly with severity of depression but weakly with cognitive impairment. Cognitive sequencing, set-formation and set-shifting deficits tended to associate with depression, but otherwise there was no association between cognition and depression. The results indicate dissociation of cognition and motor control in early PD which suggests that cognitive dysfunction is largely independent of frontostriatal dopamine deficiency underlying motor disability. Some, but not all, of the frontal lobe deficits of chronic disease are detectable in early, untreated PD. The pathogenesis of the cognitive deficits shown here appears to involve extrastriatal dopamine systems or non-dopaminergic pathology. Longitudinal study is necessary to determine whether increasing disease duration exacerbates the early cognitive deficits and affects new cognitive domains, in addition to producing increasing motor disability.

Platelet-derived growth factor receptor sequences important for binding of src family tyrosine kinases

We previously demonstrated that src family tyrosine kinases associate with activated platelet-derived growth factor receptors. We have now investigated the requirement for tyrosine phosphorylation sites in the human platelet-derived growth factor receptor for this binding. Tyrosine 857, but not tyrosine 751, is required for efficient association. Furthermore, even though src family tyrosine kinases associate with receptors lacking tyrosine 751, they are not activated, implying that association does not invariably lead to activation.

Regulation of CapZ, an actin capping protein of chicken muscle, by anionic phospholipids

Chicken muscle CapZ, a member of the capping protein family of actin-binding proteins, binds to the barbed end of actin filaments and nucleates actin polymerization. No regulation of the capping protein family has been described. We report that micelles of phosphatidylinositol 4,5-bisphosphate (PIP2) bind to CapZ and completely inhibit its ability to affect actin polymerization as measured by several independent assays. Higher concentrations of other anionic phospholipids also completely inhibit the activity of CapZ. Neutral phospholipids have no effect. Mixed vesicles of PIP2 with phosphatidylcholine or phosphatidylethanolamine also inhibit CapZ, but addition of Triton X-100 both prevents and reverses PIP2's inhibition of CapZ.

Isolation of an inhibitor of neutrophil function from bronchial lavage of normal volunteers

Bronchial inflammation is associated with a reduction in airway caliber. Factors that may dampen this inflammation are ill defined. We have developed a model of airway inflammation using an extract of cotton bracts (CBE). In the current study we characterize inhibitor(s) of polymorphonuclear neutrophil (PMN) function in bronchial lavage (BL) fluid and examine the relationship between inhibitor concentrations and induction of bronchial inflammation or bronchoconstriction by CBE. We report that BL obtained contains factors that inhibit PMN hydrogen peroxide production and chemotaxis to formylmethionylleucyl-phenylalanine (FMLP). In two different subject populations the relative degree of a one molecular mass inhibitor was greatest in BL from subjects that manifested little bronchoconstriction to CBE compared with other subjects. In one subject population the relative amount of this inhibitor correlated inversely with the bronchoconstricting response to CBE and a number of parameters of airway inflammation including PMN noted on bronchial biopsy after CBE instillation. Partial chemical characterization of one molecular mass BL-derived inhibitor reveals it is of low molecular mass (less than 1,000 Da), nonpolar, and sensitive to aminopeptidase digestion. The finding that the bronchial environment contains variable concentrations of an oligopeptide that inhibits PMN function has important implications for treatment of inflammatory airway diseases.

Functions of the major tyrosine phosphorylation site of the PDGF receptor beta subunit

Two tyrosine phosphorylation sites in the human platelet-derived growth factor receptor (PDGFR) beta subunit have been mapped previously to tyrosine (Y)751, in the kinase insert, and Y857, in the kinase domain. Y857 is the major site of tyrosine phosphorylation in PDGF-stimulated cells. To evaluate the importance of these phosphorylations, we have characterized the wild-type (WT) and mutant human PDGF receptor beta subunits in dog kidney epithelial cells. Replacement of either Y751 or Y857 with phenylalanine (F) reduced PDGF-stimulated DNA synthesis to approximately 50% of the WT level. A mutant receptor with both tyrosines mutated was unable to initiate DNA synthesis, as was a kinase-inactive mutant receptor. Transmodulation of the epidermal growth factor receptor required Y857 but not Y751. We also tested the effects of phosphorylation site mutations on PDGF-stimulated receptor kinase activity. PDGF-induced tyrosine phosphorylation of two cellular proteins, phospholipase C gamma 1 (PLC gamma 1) and the GTPase activating protein of Ras (GAP), was assayed in epithelial cells expressing each of the mutant receptors. Tyrosine phosphorylation of GAP and PLC gamma 1 was reduced markedly by the F857 mutation but not significantly by the F751 mutation. Reduced kinase activity of F857 receptors was also evident in vitro. Immunoprecipitated WT receptors showed a two- to fourfold increase in specific kinase activity if immunoprecipitated from PDGF-stimulated cells. The F751 receptors showed a similar increase in activity, but F857 receptors did not. Our data suggest that phosphorylation of Y857 may be important for stimulation of kinase activity of the receptors and for downstream actions such as epidermal growth factor receptor transmodulation and mitogenesis.

Effect of albumin on hydraulic conductivity of pulmonary artery endothelial monolayers

We tested the hypothesis that albumin reduces the vascular wall hydraulic conductivity by an interaction with the endothelium. The system consisted of luminal and abluminal chambers separated by a microporous filter onto which was grown a confluent monolayer of ovine pulmonary artery endothelial cells. The abluminal chamber filtrate was collected for timed periods during increases in transendothelial pressures of 5, 10, 15, and 20 cmH2O. The transendothelial water flux was linearly related to the hydrostatic pressure. Hydraulic conductivity (Lp) was determined from the slope of this relationship per unit surface area. In the absence of albumin, Lp of the endothelium and the filter was 14.8 +/- 3.8 x 10(-5) cm.s-1.cmH2O-1. The addition of either 2.5 or 5.0 mg/ml ovine serum albumin to the medium reduced Lp values similarly to 2.0 +/- 0.3 x 10(-5) and 2.5 +/- 1.1 x 10(-5) cm.s-1.cmH2O-1, respectively. Removal of albumin from the media reversed the effect of albumin on Lp. The filter Lp value of 3.2 +/- 0.3 x 10(-3) cm.s-1.cmH2O-1 was unaffected by albumin. Endothelial Lp value did not decrease with 5.0 mg/ml of 70-kDa neutral dextran. Albumin decreased Lp in the presence of epsilon-amino-caproic acid to the same extent as albumin alone, suggesting that the positively charged lysine sites on albumin did not mediate the effect. The results indicate that albumin decreases Lp due to an interaction between albumin and the endothelial cell.

Platelet-derived growth factor (PDGF) stimulates PDGF receptor subunit dimerization and intersubunit trans-phosphorylation

High affinity binding of platelet-derived growth factor (PDGF) has been proposed to involve the interaction of the dimeric PDGF ligand with two receptor subunits, designated alpha and beta. We have cloned and expressed a human PDGF receptor cDNA which differs in sequence from the beta-subunit and which has the PDGF binding properties and monoclonal antibody recognition, predicted for the alpha-subunit. Scatchard analysis indicated that PDGF-AA and PDGF-AB bound to transfected alpha-subunits with affinities of Kd = 0.06 and 0.05 nM, respectively. PDGF-BB bound with a significantly lower affinity (Kd = 0.4 nM). Nevertheless, this affinity is still great enough to mediate substantial PDGF-BB binding at physiological concentrations and would be considered to be "high affinity." We have used wild-type and kinase-inactive human beta-subunits to show that PDGF binding promotes receptor subunit dimerization in intact cells. In addition, we found that PDGF stimulates tyrosine phosphorylation of the kinase-inactive beta-subunit when it is expressed with alpha-subunits. The kinase-inactive beta-subunits were phosphorylated at tyrosine 857 and 751, the major phosphorylation sites of the wild-type beta-subunit, indicating either that intra- and intermolecular phosphorylation occurs on the same sites, or that a significant fraction of receptor tyrosine phosphorylation is intermolecular.

Cloning of a complementary DNA for a protein-tyrosine kinase that specifically phosphorylates a negative regulatory site of p60c-src

The protein-tyrosine kinase activity of the proto-oncogene product p60c-src is negatively regulated by the phosphorylation of a tyrosine residue close to the C terminus, tyrosine 527. The phosphorylation might be catalysed by a so-far-unidentified tyrosine kinase, distinct from p60c-src. Recently we purified a protein-tyrosine kinase that specifically phosphorylates tyrosine 527 of p60c-src from neonatal rat brain. We have now confirmed the specificity of this enzyme by using a mutant p60c-src that has a phenylalanine instead of tyrosine 527, and cloned a complementary DNA that encodes the enzyme. The enzyme is similar to kinases of the src family in that it has two conserved regions, Src-homology regions 2 and 3, upstream of a tyrosine kinase domain. The amino-acid identity of each region is no more than 47%, however, and the enzyme lacks phosphorylation sites corresponding to tyrosines 416 and 527 of p60c-src and has no myristylation signal. These results suggest that this protein-tyrosine kinase, which might negatively regulate p60c-src, represents a new type of tyrosine kinase.

Tyrosine phosphorylation and activation of homologous protein kinases during oocyte maturation and mitogenic activation of fibroblasts

Meiotic maturation of Xenopus and sea star oocytes involves the activation of a number of protein-serine/threonine kinase activities, including a myelin basic protein (MBP) kinase. A 44-kDa MBP kinase (p44mpk) purified from mature sea star oocytes is shown here to be phosphorylated at tyrosine. Antiserum to purified sea star p44mpk was used to identify antigenically related proteins in Xenopus oocytes. Two tyrosine-phosphorylated 42-kDa proteins (p42) were detected with this antiserum in Xenopus eggs. Xenopus p42 chromatographs with MBP kinase activity on a Mono Q ion-exchange column. Tyrosine phosphorylation of Xenopus p42 approximately parallels MBP kinase activity during meiotic maturation. These results suggest that related MBP kinases are activated during meiotic maturation of Xenopus and sea star oocytes. Previous studies have suggested that Xenopus p42 is related to the mitogen-activated protein (MAP) kinases of culture mammalian cells. We have cloned a MAP kinase relative from a Xenopus ovary cDNA library and demonstrate that this clone encodes the Xenopus p42 that is tyrosine phosphorylated during oocyte maturation. Comparison of the sequences of Xenopus p42 and a rat MAP kinase (ERK1) and peptide sequences from sea star p44mpk indicates that these proteins are close relatives. The family members appear to be tyrosine phosphorylated, and activated, in different contexts, with the murine MAP kinase active during the transition from quiescence to the G1 stage of the mitotic cell cycle and the sea star and Xenopus kinases being active during M phase of the meiotic cell cycle.

Comparison of 6 and 8 hourly tobramycin dosing intervals in treatment of pulmonary exacerbations in cystic fibrosis patients

The efficacy and toxicity of a shortened tobramycin dosing interval in the treatment of exacerbations of Pseudomonas aeruginosa pulmonary infection in cystic fibrosis patients were evaluated prospectively. Patients ages 13 to 30 years received 34 treatment courses and were randomized by pairs to receive tobramycin administered either every 6 or 8 hours. Peak serum concentrations were adjusted to 8 to 10 micrograms/ml; thus a larger total daily dosage was administered to patients receiving tobramycin every 6 hours. The shorter dosing interval was associated with better pulmonary function at follow-up and significantly longer time before next hospital admission for a pulmonary exacerbation. During the study hospitalization there were no differences in pulmonary function tests, clinical score, sputum carriage of P. aeruginosa, toxicity or necessary length of hospitalization. A 6-hour tobramycin dosing interval was more efficacious than an 8-hour dosing interval in the treatment of cystic fibrosis patients.

Lack of correlation between changes in polyphosphoinositide levels and actin/gelsolin complexes in A431 cells treated with epidermal growth factor

The polyphosphoinositides, PIP and PIP2, have been proposed to regulate actin polymerization in vivo because they dissociate actin/gelsolin complexes in vitro. We tested this hypothesis by comparing the ability of EGF and bradykinin to affect PI metabolism and the actin cytoskeleton in A431 cells. EGF, but not bradykinin, was found to induce ruffling and dissociation of actin/gelsolin complexes in these cells. However, both EGF and bradykinin stimulated the accumulation of inositol phosphates in [3H]inositol-labeled cells indicating that stimulation of PI turnover is not sufficient for the induction of changes in actin/gelsolin complex levels. EGF stimulated a twofold increase in the level of PIP in A431 cells. Other phosphoinositide levels were not markedly altered. Treatment of the cells with cholera toxin abrogated the EGF-induced rise in PIP levels without altering the dissociation of actin from gelsolin. These data indicate that increases in PIP and/or PIP2 are not necessary for dissociation of actin/gelsolin complexes. Overall, these experiments suggest that in A431 cells, the effects of EGF on the actin cytoskeleton are unlikely to be mediated through changes in PIP or PIP2 levels.

Plasma factor VII is activated by postprandial triglyceridaemia, irrespective of dietary fat composition

Nine adults took two 7-day diets of standardised energy and total fat content, but with a dietary polyunsaturated/saturated fat ratio of less than 0.3 and greater than 3.0 respectively, while adhering to their daily routine. Blood was drawn on 6 occasions between 09.00 and 22.45 h on the final day of each dietary period for factor VII activity (VIIc), factor VII antigen (VIIag) and lipoprotein lipid concentrations. Diurnal variation was described for each variable in terms of its deviation from the individual's daily mean value at each time point across the day. Plasma triglyceride remained low until after the midday meal, whereafter a marked rise was sustained into the later evening. Plasma VIIc declined until early afternoon, but showed a marked rise in the late afternoon. Plasma VIIag showed no significant diurnal variation. Changes in plasma triglyceride concentration during the day were related positively to changes in VIIc about 160 min later, but not to VIIc at other time points. This effect of postprandial triglyceridaemia on VIIc persisted after allowance for the effect of VIIag on VIIc. Dietary fat composition did not influence VIIc or VIIag. The results suggested an acute but evanescent effect of triglyceride-rich lipoproteins on the reactivity of factor VII, irrespective of their lipid core composition.

Variant cDNAs encoding proteins similar to the alpha subunit of chicken CapZ

Chicken adult muscle and liver cDNA libraries were screened with a cDNA, alpha 1, previously isolated from a chicken embryo library by screening with antibodies against the alpha subunit of chicken CapZ. cDNAs with a new coding region, called alpha 2, were found in addition to ones with the alpha 1 coding region. alpha 2 predicts a protein sequence that matches exactly the N-terminal sequence of 5 peptides prepared from CapZ alpha purified from chicken muscle, while the protein sequence predicted by alpha 1 matches the peptides well, but not exactly. The predicted protein sequences of alpha 1 and alpha 2 are very similar to each other, and they are similar to those of the alpha subunit of capping protein from Dictyostelium [Hartmann et al., J. Biol. Chem. 163:5254-5254, 1989] and an actin-binding protein from Xenopus [Ankenbauer et al., Nature 342:822-824, 1989]. Other conserved features of the predicted primary and secondary structures are noted. Chicken alpha 1 and alpha 2 are transcribed in all of 7 adult chicken muscle and non-muscle tissues in comparable amounts by Northern analysis. alpha 2 has four poly(A)+RNA transcripts, one of which is rare in liver. alpha 1 has two transcripts. alpha 1 and alpha 2 are encoded by different single-copy genes by Southern analysis of chicken genomic DNA.

Normal release from proactive interference in untreated patients with Parkinson's disease

The relationship of release from proactive interference (PI) to set-shifting, explicit free recall and language remains controversial. We tested 56 patients with Parkinson's disease (PD) who had never received medication and 37 matched normal control subjects on a test of PI release based on semantic category. The PD group showed normal PI release but impaired word recall. PI release was independent of impaired Wisconsin card-sorting test performance, language production, explicit memory, overall cognitive status and severity of depression. The results indicate dissociation between ability to benefit from semantic stimulus properties and processes of explicit memory, set-shifting and expressive language.

Atypical presentation of pulmonary embolism

Today we have discussed an interesting patient with an atypical presentation of pulmonary embolism. We have outlined a suggested algorithm to aid in the diagnosis and management of this disease. References 8 through 24 in the reference section are suggested readings that offer further insight into the diagnosis and management of this entity.

Phosphorylation of the PDGF receptor beta subunit creates a tight binding site for phosphatidylinositol 3 kinase

The beta subunit of the platelet derived growth factor receptor (PDGFR) coprecipitates with a phosphatidyl-inositol 3 kinase activity (PI3K) following stimulation of cells by PDGF. Mutagenesis of a tyrosine (Y) phosphorylation site, Y751, in the PDGFR, greatly reduces PI3K, consistent with the possibility that phosphorylation of Y751 signals association of PI3K. To test this we have reconstituted the binding of the PDGFR beta subunit and PI3K in vitro. Binding is rapid, saturable and requires phosphorylation of the PDGFR at Y751, but does not require PDGF-dependent phosphorylation of PI3K. To test which portions of the PDGFR are important for binding, we used an antibody to a small region of the receptor that includes Y751. This antibody blocked in vitro binding of PI3K to the receptor, while an antiserum to the C-terminus of the receptor had no effect on binding of PI3K. In addition, we found that PDGF stimulation of a cell results in the association of essentially all the PI3K activity with cellular PDGFRs. These data suggest that PI3K is a specific ligand for PDGF receptors that are phosphorylated at Y751.

Properties of tripartite chimeras between Src and Lck

To evaluate the possible regulatory effects of interactions between different domains of Src-family kinases, we have assembled five chimeric molecules containing parts of p60c-src (Src) and p56lck (Lck). Chimeras contained the N-terminal portion, kinase domain or C-terminal tail from either of the parent molecules. None of the four full-length hybrid proteins induced morphological transformation of NIH3T3 cells, nor stimulated phosphorylation of cell proteins on tyrosine residues, suggesting that their protein-tyrosine kinase activities were repressed appropriately in fibroblastic cells. However, two hybrid proteins, SLS and SLL, containing the Src N-terminal region, Lck kinase domain, and either Src or Lck tail, respectively, inefficiently induced anchorage-independent growth. Sites of phosphorylation in these molecules were determined by two-dimensional peptide mapping. SLS and SLL were both phosphorylated at their C-terminal tyrosine residues similarly to the parental molecules. Curiously, both proteins were also significantly phosphorylated at tyrosine 416, a feature of transforming variants of Src and Lck. A modified hybrid, SLO, containing the N-terminal region of Src, kinase domain of Lck and a truncated C-terminus, fully transformed cells and stimulated phosphorylation of cell proteins at tyrosine. Comparison of SLS, SLL and SLO suggests that the full-length hybrids are repressed by their C-terminal phosphorylated tyrosine residues in vivo. Consistent with this, SLO resembled activated Src in being cytoskeletal, and SLS and SLL resembled nontransforming Src and were not cytoskeletal. Hybrids with an Lck N-terminal region were cytoskeletal, like Lck itself, even though they were not transforming, suggesting that cytoskeletal localization of Lck is determined through its N terminus. Curiously, the hybrid molecules appeared not to be regulated in vitro. The specific activities of SLS, SLL and Src were approximately equal, but the specific activity of SLO was not increased, being much less than that of activated Src. Enzymatic dephosphorylation stimulated the in vitro kinase activity of parental Src, but not of SLS or SLL. These observations suggest that chimeric Src-Lck molecules are regulated in the cell, but not in vitro.

Acidic residues at the carboxyl terminus of p60c-src are required for regulation of tyrosine kinase activity and transformation

Protein phosphorylation sites act to transduce signals into changes in enzymatic activity, representing a point of interaction within a regulatory pathway. The amino acid sequence surrounding a phosphorylation site may well have several functions, including recognition by an appropriate kinase. By generating random mutations in its immediate vicinity, we have examined the sequence requirements of a regulatory tyrosine phosphorylation site, Tyr527, in the proto-oncogene product, p60c-src. The transforming and kinase activities of p60c-src are repressed by phosphorylation of Tyr527. Mutations were made around Tyr527 without changing Tyr527 or the kinase domain. Twenty-nine mutants were sequenced and classified as transforming or nontransforming for Rat-2 cells. Nontransforming mutants contained a surprising variety of COOH-terminal mutations, although acidic residues were present at positions 518 and 524 in all nontransforming mutants. Transforming mutants that contained single-residue changes at Asp518 and Ser522 demonstrated the importance of these residues. Other transforming mutants contained two or more substitutions, but the results are most simply explained if residues Glu524 and Thr523 are also important for normal regulation. Transforming mutations reduced the phosphorylation of Tyr527. We conclude that only a few of the residues in the COOH terminus other than Tyr527 are required to ensure normal phosphorylation and repression of activity in fibroblasts. Other residues may have been conserved during evolution to permit normal function and regulation in other cell types.

Macrophage colony-stimulating factor-induced tyrosine phosphorylation of c-fms proteins expressed in FDC-P1 and BALB/c 3T3 cells

The c-fms protein is a receptor for macrophage colony-stimulating factor (M-CSF) with intrinsic protein-tyrosine kinase activity. We investigated the tyrosine phosphorylation of murine c-fms proteins expressed from a retroviral vector in factor-dependent myeloid FDC-P1 cells and in BALB/c 3T3 fibroblasts transformed by the expression of the c-fms gene. FDC-P1 cells expressing c-fms were able to grow and differentiate in response to M-CSF. Their c-fms proteins were normally phosphorylated on serine and became phosphorylated on tyrosine residues contained in five tryptic peptides when the cells were exposed to M-CSF. A subset of these peptides was constitutively phosphorylated in BALB/c cells expressing c-fms, consistent with the production of M-CSF by these cells. All the peptides detected in vivo were also phosphorylated in vitro. These peptides were analyzed by susceptibility to proteases, comparison with synthetic peptides, and site-directed mutagenesis. The identities of four of the tryptic peptides were determined; they arise from three unique tyrosine phosphorylation sites. One major site of tyrosine phosphorylation at residue 697 accounted for two of the tryptic peptides. A second major site was identified at tyrosine residue 706. These two tyrosine phosphorylation sites are located within the tyrosine kinase insert region. Tyrosine 807, which has homology to the major autophosphorylation site of the p60v-src tyrosine kinase, is a minor autophosphorylation site. Possible functional roles for these phosphorylations of the c-fms protein include interactions with substrate proteins, catalytic activity, and ligand-induced degradation.