  We have observed the z=4.3 protocluster SPT2349-56 with ATCA with the aim of
detecting radio-loud active galactic nuclei (AGN) amongst the ~30 submillimeter
galaxies identified in the structure. We detect the central complex of SMGs at
2.2\,GHz with a luminosity of L_2.2=(4.42pm0.56)x10^{25} W/Hz. The ASKAP also
detects the source at 888 MHz, constraining the radio spectral index to
alpha=-1.6pm0.3, consistent with ATCA non-detections at 5.5 and 9GHz, and
implying L_1.4(rest)=(2.4pm0.3)x10^{26}W/Hz. This radio luminosity is about 100
times higher than expected from star formation, assuming the usual FIR-radio
correlation, which is a clear indication of an AGN driven by a forming
brightest cluster galaxy (BCG). None of the SMGs in SPT2349-56 show signs of
AGN in any other diagnostics available to us (notably 12CO out to J=16,
OH163um, CII/IR, and optical spectra), highlighting the radio continuum as a
powerful probe of obscured AGN in high-z protoclusters. No other significant
radio detections are found amongst the cluster members, consistent with the
FIR-radio correlation. We compare these results to field samples of radio
sources and SMGs, along with the 22 SPT-SMG gravitational lenses also observed
in the ATCA program, as well as powerful radio galaxies at high redshifts. Our
results allow us to better understand the effects of this gas-rich, overdense
environment on early supermassive black hole (SMBH) growth and cluster
feedback. We estimate that (3.3pm0.7)x10^{38} W of power are injected into the
growing ICM by the radio-loud AGN, whose energy over 100Myr is comparable to
the binding energy of the gas mass of the central halo. The AGN power is also
comparable to the instantaneous energy injection from supernova feedback from
the 23 catalogued SMGs in the core region of 120kpc projected radius. The
SPT2349-56 radio-loud AGN may be providing strong feedback on a nascent ICM.
