REGULATORY SUBUNITS OF CAMP-DEPENDENT PROTEIN-KINASES ARE DEGRADED AFTER CONJUGATION TO UBIQUITIN - A MOLECULAR MECHANISM UNDERLYING LONG-TERM SYNAPTIC PLASTICITY

In Aplysia, behavioral sensitization of defensive reflexes and the und erlying presynaptic facilitation of sensory-to-motor neuron synapses l asts for several minutes (short term) or days to weeks (long term). Sh ort-term sensitization has been explained by modulation of ion-channel function through cAMP-dependent protein phosphorylation. Long-term fa cilitation requires additional molecular changes including protein syn thesis. A key event is the persistent activation of the cAMP-dependent protein kinase at baseline concentrations of cAMP. This activation is due to selective loss of regulatory (R) subunits of PKA without any c hange in catalytic (C) subunits. To understand the molecular mechanism s that produce the loss of R subunits in long-term facilitation, we in vestigated how R subunits are degraded in extracts of Aplysia nervous tissue and in rabbit reticulocyte lysates. Degradation of Aplysia R su bunits requires ATP, ubiquitin, and a particulate component that appea rs to be the proteasome complex. Degradation is blocked by hemin, whic h causes the accumulation of high molecular weight derivatives of R su bunits that are likely to be ubiquitin conjugates of R subunits and in termediates in the degradative pathway. We also show that vertebrate R (I) and R(II) subunits can be degraded through the ubiquitin pathway. We suggest that degradation is initiated by cAMP, which causes the hol oenzyme to dissociate and, further, that the altered R-to-C ratio in A plysia sensory neurons is maintained in long-term facilitation by newl y synthesized proteins that help target R subunits for accelerated deg radation.