ATLANTA - Yolanda King, the firstborn child of the first family of the civil-rights movement, who honored that legacy through acting and advocacy, died late Tuesday. She was 51.

The daughter of the Rev. Martin Luther King Jr. and Coretta Scott King died in Santa Monica, Calif. Family members did not know the cause of death, but suspected a heart problem.

"This is just the last thing and the last person that we expected this to happen to," said Issac Newton Farris, the Kings' cousin and CEO of the King Center. "At least with my aunt [Coretta Scott King] we had some warning. Yolanda as far as we knew was healthy and certainly happy."

Former Mayor Andrew Young, a lieutenant of her father's who has remained close to the family, said King was going to her brother Dexter's home when she collapsed in the doorway. Farris said she died near Dexter King but would not elaborate.

Yolanda King, who lived in California, appeared in numerous films, including "Ghosts of Mississippi," and played Rosa Parks in the 1978 miniseries "King." She also ran a production company.

"She was an actress, author, producer, advocate for peace and nonviolence, who was known and loved for her motivational and inspirational contributions to society," the King family said in a statement.

"She used her acting ability to dramatize the essence of the movement," said Rep. John Lewis, D-Ga., who worked alongside King's father. "She could motivate and inspire and tell the story. I heard her recite 'I Have a Dream' on several occasions. She made it real, made it part of her. I think her father would've been very, very proud of her."

Yolanda King's death came less than a year and a half after Coretta Scott King died in January 2006 after battling ovarian cancer and the effects of a stroke. Her struggle prompted her daughter to become a spokeswoman for the American Heart Association, raising awareness, especially among blacks, about stroke.

(Yolanda King last spoke on the organization's behalf on Saturday at a "Gospel-Music Night for Stroke Awareness" at St. Mary Medical Center in Langhorne, Pa.)

Yolanda Denise King - nicknamed Yoki by the family - was born Nov. 17, 1955, in Montgomery, Ala., where her father was then preaching.

She was just 2 weeks old when Rosa Parks refused to give up her seat on a bus there, leading to the Montgomery bus boycott spearheaded by her father.

When the family's house was firebombed eight weeks later, she and her mother were at home but were not hurt.

She was a young girl during her father's famous stay in the Birmingham, Ala., jail.

She was 12 when he was assassinated in Memphis, Tenn., in 1968.

"She lived with a lot of the trauma of our struggle," said the Rev. Jesse Jackson, who worked with her father. "The movement was in her DNA."

The Rev. Al Sharpton called King a "torch-bearer for her parents and a committed activist in her own right."

White House press secretary Tony Snow said President Bush and the first lady were sad to learn of King's death, adding, "Our thoughts are with the King family today."

Yolanda King founded and led Higher Ground Productions, billed as a "gateway for inner peace, unity and global transformation." On her company's Web site, she described her mission as encouraging personal growth and positive social change.

"She didn't want to be a child of the movement, she wanted to be what God wanted her to be," Young said.

"She could never escape being a child of the movement, though. She was really feeling that she didn't just want to be the daughter of Coretta and Martin King. That was her struggle."

King was a 1976 graduate of Smith College in Northampton, Mass., where she majored in theater and Afro-American studies. She also earned a master's degree in theater from New York University.

Yolanda King was the most visible of the four children during this year's Martin Luther King Day in January, the first since her mother's death.

When asked by the Associated Press at that event how she was dealing with the loss of her mother, she responded: "I connected with her spirit so strongly. I am in direct contact with her spirit, and that has given me so much peace and so much strength."