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Condensin. (A) Condensin is a five-subunit protein complex that resembles cohesin (see Figure 17–23). The ATPase head domains of its two major subunits, Smc2 and Smc4, are held together by a linker protein called Brn1, which is associated with two additional proteins called Ycs4 and Ycg1. (B) These diagrams illustrate a mechanism by which condensin might generate DNA loops, thereby promoting the compaction of a chromosome. For an alternative model, see Figure 4–56. In the model shown here, the process begins when the Ycg1 subunit interacts tightly with a strand of chromosome DNA, anchoring the condensin firmly in place on the DNA. The nearby DNA curls around to interact with the hinge domain. By mechanisms that remain unclear, the hinge and ATPase domains work together, using energy from ATP hydrolysis, to move leftward along the top DNA strand (red arrow), thereby generating a chromosome loop. Such loops are common structural elements in chromosome packaging (discussed in Chapter 4).