Fig. 2

A proposed size-sensing mechanism based on microtubule motors and local translation. Kinesin motors (K) transport mRNAs associated with the RNA binding protein nucleolin (Nucl) from the microtubule organizing center (MOC) to the periphery of the cell. Upon arrival at the cell cortex, nucleolin-associated mRNAs undergo local translation. Localized synthesis of importin β1 (β), mTOR, and other proteins enables formation of a retrogradely transported complex with an importin α bound to dynein (D). Restriction of the complex to the cell center shifts protein synthesis locales from the periphery to the center of the cell [57]. Computational modeling of this system, incorporating a still hypothetical negative feedback loop at the cell center (dashed lines), suggests that it generates a fluctuating retrograde signal, the frequency of which changes with cell length or size [51]. Definitive support for this model will require elucidating the nature of the negative feedback loop and determining how the frequency encoded signal affects biosynthesis and metabolism to regulate cell size