  We present ALMA observations at 107.291 GHz (band 3) and 214.532 GHz (band 6)
of GMASS 0953, a star-forming galaxy at z=2.226 hosting an obscured AGN that
has been proposed as a progenitor of compact quiescent galaxies (QG). We
measure for the first time the size of the dust and molecular gas emission of
GMASS 0953 that we find to be extremely compact ($\sim$1 kpc). This result,
coupled with a very high ISM density (n$\sim$10$^{5.5}$ cm$^{-3}$), a low gas
mass fraction ($\sim$0.2) and a short gas depletion timescale ($\sim$150 Myr)
imply that GMASS 0953 is experiencing an episode of intense star-formation in
its central region that will rapidly exhaust its gas reservoirs, likely aided
by AGN-induced feedback, confirming its fate as a compact QG. Kinematic
analysis of the CO(6-5) line shows evidence of rapidly-rotating gas
($V_{rot}$=320$^{+92}_{-53}$ km s$^{-1}$), as observed also in a handful of
similar sources at the same redshift. On-going quenching mechanisms could
either destroy the rotation or leave it intact leading the galaxy to evolve
into a rotating QG.
