All RBs are not alike: insight into RB in the plant cell cycle

By identifying and functionally characterizing an RB homolog in a simple green algae, scientists have shed a surprising new light on the potential role of RB-like proteins in the plant cell cycle.

The retinoblastoma protein (RB) is an intensely studied protein, owing to its role as a tumor suppressor in humans. RB-like proteins exist in plants, and their function has been base on the mechanism of action in animal cells. Published in Genes& Development, this work provides an alternative model of RB action.

In mammalian cells, RB is a key checkpoint regulator of cell cycle progression. Under certain conditions, RB prevents the cell cycle transition from G1 phase to S phase (when DNA replication takes place) and can thereby help to prevent cells from becoming cancerous. When Dr. Goodenough and colleagues discovered an RB homolog in the unicellular green algae, Chlamydomonas, even they were surprised at its effects.

Despite close sequence similarities, these RBs are not alike. The RB homolog in algae does function in cell division regulation, but not as a negative regulator of S phase transition. Dr. Goodenough and colleagues found that the RB homolog functions in algae as a size-dependent repressor of cell cycle progression.

The Chlamydomonas cellular life cycle entails a prolonged G1 phase during which the cell grows to many times its original size, followed by multiple rounds of alternating S phase and cell division (mitosis). This multiple fission mechanism results in multiple daughter cells of uniform size. Implicit in this process are two fundamental cellular decisions: when to divide and how many round of division to undergo. Dr. Goodenough and colleagues demonstrate that cells deficient in this RB homolog make bad decisions.

Mutant algae cells begin dividing when they are at a premature size, and undergo an excessive number of cell divisions, producing abnormally small daughter cells. However, unlike mammalian RB mutants, these algae cells do not have a shortened G1 phase or duplicate their DNA prematurely.

This work represents the first discovery of an RB homolog in a unicellular organism, as well as the initial demonstration of how the widely used multiple fission mechanism is regulated. By discovering this unprecedented role for an RB homolog in member of the land plant lineage, Dr. Goodenough and colleagues have provided preliminary evidence to suggest that plant RBs may be less familiar than were originally suspected.