Four days after filing for divorce from Tom Cruise, Katie Holmes got back to work Monday in NYC.

Looking noticeably somber in long-sleeved navy blue dress, the 33-year-old actress stepped out for the first time since announcing her split to film a guest-judge appearance on Project Runway. Holmes' wedding ring was conspicuously noticeably missing as she made her way to Parsons School of Design in downtown Manhattan.

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A source tells Us Weekly, "She looked really sad and was looking at the ground the whole time."

Holmes called it quits with Cruise, 49, after five years of marriage and one child together, daughter Suri Cruise, 6.

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First breaking the news, Holmes' attorney told Us, "Katie's primary concern remains, as it always has been, her daughter's best interest. This is a personal and private matter for Katie and her family."

"Kate has filed for divorce and Tom is deeply saddened and is concentrating on his three children. . . Please allow them their privacy," a rep for the Rock of Ages star told Us Weekly Friday. Cruise and his second wife Nicole Kidman share kids Conor, 17, and Isabella, 19.

PHOTOS: Katie's red carpet evolution

As Us previously reported, Holmes prepared for her surprise filing by secretly renting a $12,000-month apartment on her own in NYC. For the past couple weeks, she's been quietly moving into the residence. 

"This is about protecting her daughter," an insider explains to the new issue of Us Weekly, out now. "She wants to be in charge of how Suri is being raised and didn't want her to have an exclusively Scientology education."

This article originally appeared on FIRST PICS: Somber Katie Holmes Steps Out Without Wedding Ring