Monday, October 19, 2009

Why the Golgi apparatus looks so funny


Did you ever wonder why the Golgi apparatus looks so odd, compared to other membranous organelles of the cell?  I mean, really, wouldn't you think that the forces causing other membrane-bound structures to form more of a globular shape would cause the cisternae (sacs) of the Golgi apparatus to be more, well, round?

A few days ago, the journal Cell published an article that answers that question . . . revealing an elegant mechanism resulting from the primary function of the Golgi apparatus.

As we know, the Golgi apparatus "processes and packages" proteins that arrive from the endoplasmic reticulum (ER) by way of ER vesicles. The central structure of the organelles is the Golgi stack or dictyosome, which resembles a stack of hollow pancakes. Vesicles pinch off of the first cisterna (cis face) and move to the next cisterna, then the next, and finally to the final cisterna (trans face).  Then a vesicle pinches off and moves to the plasma membrane, where it fuses and releases (secretes) it contents to the outside of the cell (exocytosis). Click here for a simplified video summary.

The new data suggest that the budding of vesicles and their movement toward the plasma membrane rely on the function of a protein called GOLPH3.  This tiny protein connects special phospholipid molecules [PtdIns(4)P] in the Golgi membrane to myoglobin molecules (MYO18A).  The myoglobin, in turn, is attached to F-actin filaments of cytoskeleton.  Well, you know what that means, right?  Yes . . . the myoglobin is a motor molecule that pulls the attached Golgi membrane along the F-actin filament, stretching it out into its familiar elongated shape.  Then thwap! . . . a vesicle pinches off and is carried away.

In short, the Golgi membranes flatten out because they are being pulled outward by the cytoskeleton in a process that produces budding of vesicles.  As simple as that!  Now, when you're describing this amazing little organelle in your A&P class, you have a new little twist to add to the story! 


By the way, the terms Golgi complex and Golgi apparatus, which are synonyms, are among the rare eponyms that appear in theTerminologia Histologica (TH). As you recall, the TH is the "official" list of microscopic anatomy terms produced by the FICAT (Federative International Committee on Anatomical Terminology).  It is named for its discoverer Camillo Golgi, who was ridiculed for believing it to be a distinct organelle.

(For a video on international terminology that you can share with students, go to http://www.youtube.com/watch?v=94PU6J3Y9mA)

Now for the next question to be answered . . . what mechanism pulled Golgi's mustache out into that crazy handlebar shape?
 
GOLPH3 bridges phosphatidylinositol-4-phosphate and actomyosin to stretch and shape the golgi to promote budding. 
Dippold, H.C. et al. 
Cell 139 (Oct. 16) 2009. 
DOI 10.1016/j.cell.2009.07.052
[The original paper. The "supplemental material" icludes a video showing the stretching of the Golgi]

Golgi's Job Stretches it Thin
Lisa Grossman
Science News October 19, 2009
[Summary article explains the context and importance of the discovery]

[For more FREE images of the Golgi apparatus, visit the FREE Image Library at The A&P Professor website.]

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