How cells know the size of their organelles

Abstract

Cells have developed ways to sense and control the size of their organelles. Size-sensing mechanisms range from direct measurements provided by dedicated reporters to indirect functional readouts, and they are used to modify organelle size under both normal and stress conditions. Organelle size can also be controlled in the absence of an identifiable size sensor. Studies on flagella have dissected principles of size sensing and control, and it will be exciting to see how these principles apply to other organelles.

Figure 1

Examples of organelle size control. (A) Cell specialization-The ER expands greatly from an undifferentiated cell (left) to a professional secretory cell (right). (Image from (43)) (B) Cell specialization-The outer segment in cone (COS label) and rod (ROS label) cells differ in shape and size. (C) Functional demand-Mitochondria ramify in a yeast cell grown in respiratory (right) vs. fermentative (left) media. (D) Organelle/cell size scaling-The nucleus in the larger cell (right) is larger than in the smaller cell (left).

Figure 2

Cartoons illustrating possible models for size sensing in a one-dimensional organelle, where size-sensing molecules are represented by green and red circles. Examples are shown for when the organelle (black rectangle) is growing (left column) and has reached a steady-state length (right column). The organelle grows until: (A) enough sensing molecules accumulate on the organelle; (B) the concentration gradient of the sensing molecule achieves the right profile; (C) the sensing molecule's occupancy time is long enough to induce a state change (green to red circle); (D) a conformational change in the organelle is induced by accumulated sensing molecules; (E) its size conforms to a structural scaffold set by the sensing molecule.

Figure 3

Illustration of the balance-point model of flagellar length control. (A) IFT particles (green) associated with the flagellar basal body bind precursor molecules from cytoplasmic pool (red) and are injected into the flagellum where they travel to the distal tip to deliver precursor for flagellar assembly. Flagella assembly rate depends on the rate of anterograde IFT, and the disassembly rate is constant. A length sensor may act at the flagellar tip or base or both, and it can affect gene regulation & precursor synthesis, injection of IFT, or assembly or disassembly rates to create feedback loop for length control. (B) Assuming a constant IFT speed, v, the delivery frequency of IFT particles, f, will vary inversely with flagellum length, L. (C) In the balance-point model, flagellar length reaches a steady state where the assembly rate (blue) and disassembly rate (orange) curves intersect.