Decapping complex is essential for functional P-body formation and is buffered by nuclear localization

mRNA decay is a key step in regulating the cellular proteome. Cytoplasmic mRNA is largely turned over in processing bodies (P-bodies). P-body units assemble to form P-body granules under stress conditions. How this assembly is regulated, however, remains still poorly understood. Here, we show that the translational repressor Scd6 and the decapping stimulator Edc3 act partially redundantly in P-body assembly by capturing the Dcp1/2 decapping complex and preventing it from becoming imported into the nucleus by the karyopherin ß Kap95. Nuclear Dcp1/2 does not drive mRNA decay and might be stored there as a ready releasable pool, indicating a dynamic equilibrium between cytoplasmic and nuclear Dcp1/2. Cytoplasmic Dcp1/2 is linked to Dhh1 via Edc3 and Scd6. Functional P-bodies are present at the endoplasmic reticulum where Dcp2 potentially acts to increase the local concentration of Dhh1 through interaction with Scd6 and Edc3 to drive phase separation and hence P-body formation.