The empirical relations between the central engine and the relativistic jet, and the physical processes behind these relations are still poorly understood, partly because they operate close to the BH within the central light year, which subtends sub-milliarcsecond angles from Earth. I will present (i) how the combination of the sub-milliarcsecond resolution radio observations of the jets and optical spectroscopy of nuclear regions in the host galaxies can be addressed for exploring the structure and dynamics and exact radiation mechanisms operating in the innermost of the parsec-scale region of AGN, and also (ii) the recent results on the inner structure and emission mechanism of the radio-loud galaxy 3C 390.3 infered from the six years radio (VLBI) and spectral optical monitoring of this source. We found an observational evidence for the link between variability of the radio emission of the compact jet, optical continuum emission and ejections of new jet components in the radio galaxy 3C390.3. The emission mechanism of continuum emission, structure of the sub-parsec-scale nuclear region of the 3C 390.3 and its similarity with microquasrs will be discussed.