Michael P. Murtaugh

Transglutaminase and receptor-mediated endocytosis in macrophages and cultured fibroblasts

SummaryThe function of intracellular transglutaminases remains to be clarified. In fibroblasts the links between the activity of this enzyme and receptor-mediated endocytosis are complex and open to interpretation. However, the issue cannot be firmly laid to rest until the structural specificity of the alkylamine inhibitors of endocytosis is explained. In macrophages, there is substantial evidence that the enzyme plays some role in receptor-mediated phagocytosis, but what this role is and how it might relate to endocytosis in other types of cells is at present an unresolved issue.

Revertants of a trans-dominant S49 mouse lymphoma mutant that affects expression of cAMP-dependent protein kinase

Phenotypic revertants were isolated from an S49 mouse lymphoma tissue culture cell mutant that lacks cAMP-dependent protein kinase (cA-PK) activity (kin-). The mutant phenotype is trans-dominant and results from a lesion that probably lies outside the cA-PK subunit structural genes. The nature of the event that produces the kin- phenotype is unknown. However, the mechanism that is responsible for its behavior is genetically encoded because: spontaneous revertants arise at low frequency; reversion frequency is increased by mutagen treatment; mutagen-specific classes of revertant phenotypes are induced; and some revertants are temperature-sensitive for expression of cA-PK subunit polypeptides. Additional evidence is provided that argues against structural lesions in cA-PK catalytic (C) subunits as explanatory of the kin- phenotype. Kin- cells do not express an immunologically detectable C polypeptide, whereas C expression is restored in revertant cells. Revertants in which phenotype and cA-PK activity levels are only partially restored to that of wild-type cells contain a commensurately reduced amount of C polypeptide. Finally, the structure of C polypeptide in partial revertants is unaltered from that of wild-type C. The evidence supports the hypothesis that the kin- lesion defines a regulatory gene responsible for setting intracellular levels of cA-PK C subunit expression.