WBR0847

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A researcher is studying the molecular mechanisms of cellular transport. He purifies and salt-washes organelles from squid axoplasm and observes their movement on purified microtubules using video-enhanced microscopy. Initially, the organelles exhibit a nearly undetectable level of movement despite an ATP rich microenvironment. However, addition of a high speed axoplasmic supernatant increases the movement frequency and speed more than 20-fold. The researcher also notices distinct movement of organelles from the negative pole to the positive pole of the microtubules. Which compound in the squid axoplasmic supernatant is responsible for the movement of vesicles?]]
Answer A	AnswerA::Vimentin
Answer A Explanation	AnswerAExp::Vimentin is a type III intermediate filament found in mesenchymal cells.
Answer B	AnswerB::Desmin
Answer B Explanation	AnswerBExp::Desmin is a type III intermediate filament found in muscle cells.
Answer C	AnswerC::Dynein
Answer C Explanation	AnswerCExp::Dynein is a protein that transports organelles from the positive pole to the negative pole of the microtubule.
Answer D	AnswerD::Kinesin
Answer D Explanation	AnswerDExp::Kinesin is a protein that transports organelles from the negative pole to the positive pole of the microtubule.
Answer E	AnswerE::Plasmin
Answer E Explanation	AnswerEExp::Plasmin is a protein that degrades fibrin mesh. It plays a role in the anticoagulation pathway.
Right Answer	RightAnswer::D
Explanation	[[Explanation::Transport of membrane-bound organelles (MBO) along the microtubules requires special proteins that can move organelles, like vesicles, from the positive pole of the microtubule to its negative pole and vice versa. Early experiments that studied the movement of vesicles showed that organelle transport requires not only ATP as a source of energy but also transport proteins, such as kinesin and dynein. Kinesin is capable of transporting organelles from the negative to positive pole of the microtubule; whereas dynein is capable of transporting organelles from the positive to negative pole. As organelles were initially purified and salt-washed, the transport proteins were lost, and despite an ATP rich microenvironment, no movement occurred. Addition of axoplasmic supernatant that contained the transport proteins led to eventual organelle transport along the microtubule. Educational Objective: Kinesin helps transport organelles from the (-) to (+) pole of microtubules. References: Schroer TA, Schnapp BJ, Reese TS, et al. The role of kinesin and other soluble factors in organelle movement along microtubules. J Cell Biol. 1988; 107:1785-1792.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Kinesin, WBRKeyword::Dynein, WBRKeyword::Transport, WBRKeyword::Protein, WBRKeyword::Proteins, WBRKeyword::Organelles, WBRKeyword::Microtubule, WBRKeyword::Microtubules
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