General Motor Co. tweaks its marketing ranks today, expanding the role of newly hired Chris Perry to oversee all U.S. efforts and splitting Buick-GMC responsibilities between two executives.

GM names Perry vice president-U.S. marketing, replacing Joel Ewanick, who was promoted to global chief marketing officer in December.

Perry, 50, was hired last year to guide Chevrolet marketing shortly after Ewanick came to GM from Nissan Motor Co. Ltd. Both men worked together at Hyundai Motor America Inc., which witnessed a growth spurt during their tenures.

GM says Perry’s new responsibility will be “improving the positioning of the Chevrolet, Buick, GMC and Cadillac brands and consumer consideration of GM vehicles in the United States.”

Longtime Chevy marketer Rick Scheidt, 57, takes over for Perry at the bow-tie division.

GM also splits marketing duties at Buick-GMC. John Schwegman, 46, will give up Buick to exclusively lead GMC, while 46-year-old Tony DiSalle will move from marketing the Chevy Volt to guiding Buick.

“These moves support our plans to continue building very clear and distinct swim lanes for our four brands,” Ewanick says in a statement.

GM has been tinkering with its divisional marketing leadership since emerging from bankruptcy in 2009, hoping to hold onto U.S. market share with four fewer brands.

The auto maker’s deliveries in 2010 grew 7.2% and its share of the U.S. market shrunk to 19.1% from 19.8%.

GM also announces today the appointment of Linda Marshall to president of OnStar, replacing Chris Preuss who has decided to leave the company.

Marshall joined GM in November as executive director-Global Business Strategy and Development at OnStar. She came from Revol Wireless, where she served as chief operating officer, and also spent time with wireless giants Sprint Nextel Corp. and Verizon Communications Inc.

GM has been trying to expand the usefulness of its OnStar telematics service beyond safety and security and into personalized options, such as owner apps for Smart phones. It also recently decided to sell the service at retail so owners of older GM vehicles and non-GM vehicles could subscribe.

Marshall will report to GM Vice Chairman Steve Girsky.

All of today’s executive appointments are effective Feb. 1.