Cytoplasmic truncation of the p55 tumour necrosis factor (TNF) receptor abolishes signalling, but not induced shedding of the receptor.

The mechanistic relationship between the signalling for the TNF effects by the human p55 TNF receptor (hu-p55-TNF-R) and the formation of a soluble form of the receptor, which is inhibitory to these effects, was explored by examining the function of C-terminally truncated mutants of the receptor, expressed in rodent cells. The 'wild-type' receptor signalled for a cytocidal effect when cross-linked with specific antibodies and exhibited spontaneous shedding. Shedding of the receptor was not affected by TNF but was markedly enhanced by 4 beta-phorbol-12-myristate-13-acetate (PMA). Receptor mutants with 53%, 83% and 96% C-terminal deletions could not signal for the cytocidal effect. Furthermore, they were found to associate with the endogenous rodent receptors, interfering with their signalling. Yet even the deletion of 96% of the intracellular domain did not abolish shedding of the receptor in response to PMA. These findings suggest that signalling and shedding of the p55 TNF-R are mechanistically distinct.